I ran the following gearings when I was running SP motors, gearing on my current Orion motors is basically the same (note this is for open astro / multi-surface tracks, in brackets for smaller indoor tracks) .
5.5 = 20 or 21 (19 on very small tracks)
6.5 = 23 or 24 (21 on very small tracks
7.5 = 26 or 27 (25 on very small tracks)
I hope that helps
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