Cnc cutting the wrong size..

[JAZZCNC]

Tom 200 is the full step count for your steppers but your drives will have micro stepping. Most drives don't allow full step and start at what's called Half (1/2)step or 400 Micro-steps per rev. Also shown as 2X on some drives IE: 2 x 200.
So the way to calculate steps Per Millimetre is to take the micro-step unit and divide by the pitch of the lead screw. IE: drives set at 800 MS on 2.5mm pitch 800/2.5 = 320 Steps per millimetre.

Regards setting or choosing the micro stepping then here's a general idea of the affect they have.
Choosing a lower MS number IE:400 will allow faster motor speeds in tuning due to less pulses required from the Parallel port but not give such smooth running of the motors. It can also cause motors suffer from resonance more has well.

Higher MS will give much smoother running of the motors but demands much more from the PP so less pulse's equal less speed in motor tuning. Doesn't suffer from resonance so much has well.
That said then there is a limit to how much MS is useful or usable and the rule of thumb is don't go much higher 10x or 2000 pulses has the motors can't resolve to much more than that and your just wasting and stressing the PP more.

Personally I prefer higher MS has rapid speed is of little concern to me and prefer smooth running motors so often use 2000.