It's a 2:1 ratio with 500mm pitch screw, so 10mm effective pitch and (I think) 1600step/rev. That means in mach3 it should say 160step/mm, or at least that's what I would have entered for you (or 80 if it's actually on 800step/rev). I can't see why it would have changed unless you accidently changed it when I suggested lowering the acceleration, but even so if the error is inconsistent something more interesting is happening.

Looks like it was a right angled/isosceles triangle as 95*300/100=285mm. If you're just using a ruler that easily accounts for the 1mm difference. I'm surprised as I checked it with digital calliper over 300mm and it was spot on.

I told Luke how to work these values out, see if he remembers

Feel free to call me too if Jazz or Luke haven't already sorted it.