I've got a small leak from n/s drive shaft oil seal on a 7" 4.1:1 Sierra/Quaiffe diff, I have new seals ready to fit.

The Sierra book says to measure the depth of the old seal with a feeler gauge before removing, then fit new seal to same depth as old seal. Seems a bit hit and miss to me and the reason I have a leak could be because it's fitted at the wrong depth. So before I strip the drive shafts off the car anyone got any better ideas or a definitive depth for fitting? Ta.