you have good title --SPOS and gear ratio

means you understand the positioning related to gear. 

you can check if your feedback (encoder ) installed on the spindle (milling head) and ratio 1:1, that will no gear factor being concerned

if your encoder is spindle motor feedback you need pay attention, but if this is professional OEM machine, the builder should already set up in the MD

another tips

#1 DON'T not run SPOS at beginning make spindle function (S1000 M3 ) for a while then use SPOS

#2 I think there are Setting Data to modify the fine position --you need make sure SOPS not stop random ,     

gtsolid

Hi everyone!

I have a pinion that should move a bigger head with a ratio 1:3 (see picture).

I'm using SPOS command and i'd like to move to 90° position writing SPOS=90 but it will move to an intermediate position. So I change DM31060 but I obtain other wrong results.

How do you think I can manage this issue?