For milling I use a 36:45 XL belt reduction. Larger pulleys cut less slack and XL pulleys are cheap from RS.

This converts a common or garden 5mm pitch screw to the slightly less usual 4mm pitch at the motor, allows NEMA34 motors without overhanging the mill bed and makes the maths easy...

400 steps/rev on a 4mm (pretend) screw is 100 steps/mm, 10 microns

Switch it 800 steps/rev and you get 5 microns which is about the best you can hope for without spending a lorra lorra money on iron :whistling:

Switch it to 1600 steps/rev, if you can sustain the pulse train for a G0. Tell the hoy poloy that it "resolves" 2.5 microns, tell everyone else that it makes it quieter :heehee: