I would think a range of 24 to 29 would do you, that would cover stock/19t up to a 10 or a 12 turn motor, go with the 78 or 81 tooth spur gear.
www.rwracing.co.uk and either email or call him, the shaft isn't up on the website.
TAM DF-03
78 81
PIN 17 14.02 14.56
18 13.24 13.75
19 12.54 13.02
20 11.91 12.37
21 11.35 11.78
22 10.83 11.25
23 10.36 10.76
24 9.93 10.3
25 9.53 9.90
26 9.16 9.52
27 8.82 9.16
28 8.51 8.84