I think you are still missing the point. If you are only looking at the error between where the telescope thinks it is - and where plate solve says it is - and correcting the slew position based on that - it will have no improvement for my mount - and others - over normal sync/slew centering. In fact - it is sync/slew centering - it's just that SGP is now doing the sync.
That will not work for my mount and it explains why it is not converging.
The goal of centering is to go to X and have it land at X. If it lands at X + dX the goal is to make dX zero.
The point of syncing is to make the telescope coordinates match the plate solve value when the telescope is not moving. That is neither necessary nor sufficient to make the dX value above go to zero.
If instead of looking at the difference between the mount's coordinates and the plate solve - you look at the difference between the slew target and the subsequent plate solve after the slew - you will see that after one iteration my mount would point with an error of a few arc-seconds - and will continue to converge in more iterations - which probably won't even be needed. It is much more accurate and much faster.
Otherwise this will have no improvement for my mount and others - that don't have a problem with syncing, and that are capable of very accurate centering when combined with a plate solve.
Frank