Servo arm spacing is 17mm. Most people are using the secraft’s aluminum arms that break in case of a crash.

The stock ratio is:

First pulley 20T, optional 21T, 22T and 23T
Second pulley: 61T
Third Pinion: 17T
Fourth Main Gear: 58T
Ratio =(61/first pulley)*(58/17)

Ratios:
20T = 10.406
21T = 9.910
22T = 9.460
23T = 9.048

You can also use a fixed main gear tooth count of 208.12 and use the standard pulley tooth count.

20 : 208.12
21 : 208.12
22 : 208.12
23 : 208.12

Because:

208.12 / 20 = 10.406
208.12 / 21 = 9.910
208.12 / 22 = 9.460
208.12 / 23 = 9.049

Avant RC.