for a stepper fitted machine, that has 400 steps per rev, and the gear reduction is 2.5:1, with the 5mm pitch you get 200 steps per unit in metric. For a 2:1 gear reduction it should be 160 steps per unit. All you have to do now is equate that to the steps per rev your motor has.

steps per rev / ( ballscrew pitch/gear reduction ) = steps per unit

If you have a bit of backlash check the angular contact bearings at the pulley end of each ballscrew, they may need adjusting.

Ps. don't forget to fold the tab washer flat to adjust the nut.