John,
My spindle speed is pretty close on the high range.
I kept changing the pulley ratio in MACH 3 config for the 2nd pulley, and trying to call it "low range".
I just ended up guessing at it and it seems to work ok for the low range.
In a program you have to change M03 to M04 if it is in low range, for forward direction.
As long as I'm the only operator, no problems.
JAckal