On a new ford racing gearset, there should be a chalk marking on the new pinion. 1, 2, 0, -1, -2, ect.
IIRC, those are for adding or subtracting (10X) thosandths of an inch to your factory pinion depth shim measurment.
For example, your in car pinion shims measure .050 of an inch. A 2 marked on the pinion would be +.020 to that measurment. So total shims would need to be .070.
Subtract the change if its a negative chalk number.
Its been a long time since I did this, and going off memory alone. Research it elsewhere. But If I remember right it sets up pinion depth almost perfectly with a new ford gearset in a factory (untouched) rearend.