be careful going to small with the gears, I originally bought a 77T spur and a 44T (64P) and the diameter of the motor prevented the gears from meshing, not enough adjustment travel. I then went to a 85T spur and a 51T pinion and that put me pretty much in the middle of the adjustment on the YR adjustable motor mount.

As long as you go with a spur around 85T (64P) you can cover a wide range of FDR's.

Cheers,
Jim