What size PSU are you using? Also, if I'm not mistaken, G201 has electronic mid band resonance damping. Might want to contact Marriss at GeckoDrive for an opinion.
The math is actually easier than that. 200 steps X 10 microsteps = 2000 microsteps. Multiply by gear ratio 2000 X 1/3 = 666.6 microsteps/rev. Dividing your lead by that gives you .00029"/step