[Chrislong]

I used to use 18/104 on a Checkpoint 10x2.

What I suggest you do is try 18/106 as a starting point, and if you want more punch go to the 108, if you want more top-end, go to a 104. Be aware that changing 2 teeth on the spur is going to effect the ratio by less than 2% so you might not notice much effect, whereas changing by 1 tooth on the pinion will effect the ratio by over 5%.