Installing Axle Housing Seals: How Deep?

'68 Dart, 8-3/4 Posi
I'm re-assembling after MANY years, so a lot is fuzzy. When installing the inner axle seals, how deep should I expect them to set? The shop manual just says to install them...no instruction. Do I just keep tapping them till they don't go any more, or is there a rule of thumb as to how far in they should go?