Ahh ****, I've been up all night. If noone else pops up with the formula today. By this afternoon, I should be able to tell you. Do you know which drive gear you have on the output shaft? That will matter. There is only 2 different ones. Maybe you can take a pic of where it enters the transmission and I should be able to tell which one. Either small gear or large gear. The Speedo housings are different for little gear and big gear on the output shaft, and what trans again?
Sometimes on your profile, where it says Engine, put your trans too and some even like to put ther rear end and ratio. Or have a a Signature with the paticulars. It really helps alot when some are trying to help. Notice mine says, 350/Th350/NP208.