Step resolution

I am wanting to calculate the resolution of my machine and using the
calculation in the mach2 instructions get a figure of .0025mm per step
working on 400 steps per mm.

the servo has 1000 pulses per rev (250 step diff encoder), 2:1 belt
reduction (26:13) and 5mm belt pitch

is the .0025mm per step right, and if so how good is this figure.
Also what would this be in imperial ?


Thanx

Aaron