A series of tests were carried out to understand the functionality of the Test Screen on the Dome Control Unit (Settings/Test) and issues related to employment of the Home Magnet. 

1. a) Test started with Dome at Parked Position (Az 90). Test Screen showed Number '13' with an Unfilled Circle. Dome moved to Az 190.
   As the dome was rotated clockwise the number value increased rapidly, and it is concluded that the number is showing Encoder Ticks and not degrees. As dome moved across its Home position (Az 179.9) the number on the Test Screen reached something like 6300-6400 (for a 90 deg change in Azimuth). Circle wasn't noticed to become filled at any stage, but as the dome was moving rapidly through the Home position its assumed that the display refresh wasn’t quickly enough to show the precise point where Circle could have become filled.
    
    b) Test started with Dome at Az 170.  Find Home used (firstly from DeviceHub, but later from the Dome Control Panel).
   Dome searched for and found its home position (Az 179.9).  It does this with a distinct behaviour whereby the dome stops slewing slightly beyond the home position after which the dome goes back a short distance stopping at the precise home position.  With the Dome at Home Position the Test screen showed number 6315 with a Filled Circle. Adjusting the dome position back-and-forth slightly showed that the interval showing a Filled Circle was around 35 ticks wide. With around 70 encoder ticks per degree (from 6315 / 89.9),  the interval with a Filled Circle corresponds to an region about 0.5 degrees wide.
    
   During later test to Find Home, the dome passed and went back to around 6317 (i.e. it had successfully found home) but the Test Screen showed an Unfilled Circle. Nudging the dome slightly brought it back into the zone where the circle was filled.
    
   The rectangular bar on the Test Screen appears to remained unchanged throughout the Test
   
   >> The Circle on the Test Screen become Filled when the Home magnet comes within range of the Encoder Arm. At other times the Circle is Unfilled.
   >> The Dome Controller seems to choose its Home Position at one particular end of the home detection range (rather the mid-point). 
   >> When dome is at it's Home position the Circle may or may not appear as Filled.
   >> The purpose of rectangular bar on the Test Screen in unknown (it doesn't appear to change during any of the tests tried).
    
2. a) Test started with Dome at Home Position.  Dome commanded to Park. 
   Number on Test screen fell rapidly as the dome rotated, and reached number '24'  at Park Position
   Power to Dome Controller turned off and after a short while turned on again.  The Test screen showed number '0'.  
   
   b) Test started with Dome at Park Position (Az 90). Dome commanded to Find Home, with the test screen not showing.
   After reaching Home, Test screen opened and found to show number '6310'
   
   >> The number on the Test screen resets to zero when Dome Controller is started.  
   >> The number shown is a running number of Encoder Tick changes  relative to value 0 when the dome controller was last powered on.
   >> The precise number doesn’t matter, once calibrated the dome uses the difference in the number between two different positions to calculate the angular difference, and when synced to calculate the dome azimuth.
   >> Not surprisingly the counting of Encoder Ticks doesn’t depend on the Test screen being open.
    
3. a) Test started with Dome at Park Position (Az 90) and with number of Encoder Ticks sitting at 0.
   Dome rotated in anticlockwise direction to Az 80. (opposite to where it would move if going directly towards Home Position (Az 179.9). Number expected to either becoming negative or to become a number like 25260 (like a clock going from 00:00 to 23:59), but it didn’t. It began incrementing (without any sign).
   
   b) Dome rotated a full 360 degs.  Number on Test Screen reached 25230+  and just increased further as dome rotated even further further round.
   
   >> So the number on Test Screen reports the net number for encoder ticks moved relative to when it was turned on.
   >> Dome controller probably keeps tracks on whether the net movement has been clockwise or anticlockwise but it doesn’t report this to the test panel.
    
4.  Home Magnet was examined and found to be corroding with it’s thin casing flaking off where it still remained.
   
   >> Current Magnet still works ok to find Home,
   >> But based on original supplied magnet, the corrosion will eventually progress to the point that it is no longer strong enough to be detected by the Dome Unit/Encoder.
   >> Corrosion issue is due to basic rusting process (metal/Fe + water + oxygen). 
   >> The magnet is inevitably exposed to water from dew/condensation created during nightime observing sessions.
   >> Neodymium magnets are apparently known for their poor resistance to corrosion, unless they are waterproof versions that are coated by a plastic layer or shrink-wrapped in a PVC cover (which the Pulsar supplied magnets are not)
   >> The position that the Home magnet needs to be placed in my system, where it sits in direct contact with metal embedded in the reinforced rubber trim, might enhance the rate of corrosion either directly by some electrolysis effect or by causing damage/scratching to the magnet's 'protective' casing.
    
5. a) Pulsar supplied magnet were temporarily removed and replaced with one of the magnets that had been previously purchased as backup. Immediately noticed the 'backup' magnet (3mm width) was a lot thinner than the Pulsar supplied magnet (5mm width). Tried to Find Home, but the dome passed the Home position without finding it. Repeated Twice, same result.
   
   Dome rotated to azimuth (179.9) corresponding to the Home Position, Circle on Test Screen was unfilled (nudged dome back-and-forth, but Circle remained reunfilled).
    
   b) Two magnets  (2 x 3mm) used instead of one (3mm).  Find Home used.  This time dome was able to successfully detect them and find home. So it seems that I can get away with keeping them as backups. (Note : with just one magnet it didn’t matter which orientation, both orientations failed).
   
   >> Single regular magnets (at least narrow ones) don't have sufficient strength to allow Home position detection
   >> Using two regular magnets together do have sufficient strength to allow Home position detection
   >> Two regular magnets may or may not prove to have better corrosion resistance than the 5mm Pulsar Supplied Neodymium magnet
    
   When testing with two magnets, it was found to be critical what orientation the magnet(s) had. When facing one way the home position could be found, but not when they faced the other way. This test was repeated wiith Pulsar supplied magnet and the same result
   
   >> Magnet facing direction is critical ! If the magnet is facing the 'wrong' way, finding home & performing calibration will be problematic.
   >> If Dome rotates continuously and can't achive a successful Calibration it might be due to the Home magnet facing the wrong direction.
   >> Dome 'Find Home' should be shown to successful before attempting a dome calibration (Test Screen can be used to demonstrate Magnet detection).

Key Findings
Key finding are also summarised on the Observatory 2018 Page in section Home Magnet Tests (2022-07-13)

Notes:
The magnet supplied by Pulsar is a 10mm diameter x 5mm thick, very strong Neodymium magnet,  like these ones from Amazon :
 F645-N52-10 Magnet Expert 10mm dia x 5mm thick Ultra High Performance N52 Neodymium Magnet - 3.2kg pull ( Pack of 10 ) by first4magnets™
 https://www.amazon.co.uk/Magnet-Expert-thick-Performance-Neodymium/dp/B00GFMQ5OM   (£10.48)

 The regular magnets that were purchased as potential backup are :
  10 x Cylindrical Magnets 3mm Thick By 10mm in Diameter Round Disc Magnets Very Strong  1.2kg pull by Harlington Group
  https://www.amazon.co.uk/dp/B08426C287/ref=pe_27063361_485629781_TE_item  (£3.95)
  (the received magnets have a better casing than the pictures & reviews on product page)

Update 2023-11-17.
After replacing one of the dome's Drive Wheels/Connectors on 2023-11-17 (which had become loose/badly worn) and before the dome was recalibrated,  the dome's home magnet was checked and was found to be almost totally corroded with minimal remaining strength and is non-functional - the dome was unable to find Home Position using it.   This was happened on two previous occasions in 2000 and 2022. The Home Magnet is believed to corrode in my Observatory due to an electrolysis effect caused by its contact with metal beading in the rubber trim attached to the edge of dome's roof flange combined with frequent wetting from dew drops running down the inside of the observatory dome. The pieces of the corroded magnet were removed and replaced by a new magnet (actually by a pair of magnets,  see Home Magnet Tests, 2022-07-13). After successful calibrating the dome the new magnets were removed and carefully stored ready for some future use - repeated experience shows that they can't be left in position for extended periods without corroding.