Final Drive Ratio is used because that number is fixed, the gearbox internal ratios do vary between different cars and you adjust the spur and pinion gears to give you the FDR. The motor/ESC manufacturer can't tell what car it is going in, but can give you what the overall gear ratio should be.
If you want an FDR of 10:1 and the internal ratio (the layshaft gear/pulley and the diff gear/pulley) is 2:1 then want a 5:1 ratio with the spur and pinion, 2 x 5 =10
If the internal ratio is 2.5:1 then you want a 4:1 ratio with the spur and pinion, 2.5 x 4 =10
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