Blimey, this is a blast from the past! I seem to remember first posting on this topic in 2001. Since then, I've moved on for instructing and flying classic jets to flying Airbus wonderjets where all you have to do to fly a NPA is press the Approach button and sit there arms folded while the kit does it all until MDA when you press the red button and land it.
But as far as I remember, at GA aircraft speeds of about 90 to 120 knots, I used to reach anticpate by about 2 degrees and for airliners about 5 degrees. This means you start to roll out when the needle has either 2 or 5 degrees before it reads the approach course.
But this theory relies heavily on the TLAR premise. (That Looks About Right). Hope this helps.