Final Drive Ratio can be calculated like this: Spur divided by Pinion multiplied by Internal (drivetrain) ratio
The Drivetrain Ratio (also called the internal ratio) on a T3 is 1.9. Drivetrain ratio is the number of teeth on the differential/spool pulleys(38 for a T3) divided by the number of teeth on the layshaft pulleys(20 for a T3).
So for your car the formula is:
Spur/Pinion X 1.9 = FDR