Fairly straightforward once you know how...
Your motors are 200 steps per rev with the drivers set on 1/8th step, this means you have 1600 steps per rev. (200 steps x 8 microsteps).
Lead screw is 2mm for 1 turn, mach3 needs to know how many steps for 1 unit (or 1mm).
So, 1600 steps will turn you leadscrew 1 full turn and in your case move your axis 2mm. Therfore 1600 steps divided by 2mm = 800 steps per unit (1mm).
You could easily get away with setting your boards to 1/4 or 1/2 stepping as set at 1/8th stepping with 2mm pitch leadscrew the accuracy will be 0.00125mm per step, change them to 1/2 stepping then each step will be 0.005mm, the less steps you need the better if you ask me... Of course if you change to 1/2 stepping then the steps per unit will need to change to 200. (400 steps per rev divided by 2mm).