Disregard steps 1 and 2, that's not safe to do if the parallel port outputs over 5v. Measure the max voltage as I said in the previous post then just desolder the switch and solder the signal output wire from the parallel port to the PCB pin of the switch which was connected to R31. Then adjust the pot untill it gives the same reading for voltage with mach3 at 100% spindle speed as it did with the pot at 100%. Good idea to remove R32 whilst you adjust the voltage, then put it back when you're sure it's right - that guarantees not to break anything. Quite tricky soldering though!