The internal ratio for the TRF419 is 1.85 and is on top of page 20 in the manual.

For my club's 17.5 blinky sedan class, I use an FDR value around 3.5 to 4.0 depending on the track layout. With that in mind, a 55T pinion and 113T spur would yield an FDR of 3.9 Adjust from there.