The internal ratio of the Pro 4 is 2.4375.
triumph, to work out the final ratio the formula is:
(Spur Size/Pinion Size) X Internal Ratio
So say you had a 100 spur, and a 25 pinion and were using a Pro 4 you would do:
100/25 = 4
4 X 2.4375 = 9.75
Final Ratio = 9.75
Hope this helps.