I just re-read the whole thread...unless I'm mistaken (could be!) I believe the settle TIME means the guide star has to stay below the error for that whole time before settling will be considered complete. So settling at 2.5px for 20 seconds would mean that the error has to stay BELOW 2.5px for all of 20sec. That's well out of the range of what Keith needs to do.
Of course getting rid of that 2.5px limitation would solve the issue completely.