It's not.Be interesting to know if the guesstimate of 3/8" is correct or not.
where the pinion gear is in relation to the mounting holes on the differential has zero to do with the pinion being offset in the vehicle. The differential does not have to be bolted in the middle of the car. The differential can and often is offset to the passenger side of the vehicle by < 1"
The difference in angles because of one of the axles being longer is going to make a < 0.01" difference in the length. And using distances that are isolated to the differential only is NOT gong to tell you if the pinion is offset from the center of the vehicle. you need a point of reference on the vehicle to measure from that is a constant like the track width. You can also use the back side of the knuckles so long as the vehicle is sitting level or of the suspension is hanging so long as the suspension hangs the same distance on both sides.
... to check pinion location relative to the two mounting bosses, which we (well, I, anyway) know are centered on the car. ...