This is a problem for any SGP user that wants accurate centering. The newer builds of TSX don't work at all with centering. They just won't get closer. Large slews work but it will not move once the correction is only a small amount. The centering error doesn't change after a certain point. I will go ahead and post a question with SB.
I use the POTH because without it, slewing of my MX+ won't work with the PHD2 drift alignment tool. TSX ASCOM driver does not pass location coordinates to PHD2. PHD2 RA slewing uses degrees from meridian so it needs to know where you are. The POTH has a place to manually enter lat/long coordinates. Other functions of SGP and PHD2 work fine without POTH but I just leave it in there.
Chris