Actually you will know that you crushed the washer when the differential fails in six months (or less - depends on how much you crush it).
As it turns out many differentials with pinion seals leaking already have a pinion bearing problem.
There is really nothing you can do about it now. Without a way to place the nut at its original position the best I could say would be to slowly tighten the nut with some locktite lubricating the threads. At the point of bottoming the nut, you should not have to go much father to be tight. If the nut doesn't torque linearly, stop at whatever position it stops being linear. All that is required of the nut is to mantain the inner race of the outer pinion bearing. The level of torque is just so it doesn't back off. With a load of locktite, it probably does not need to be as tight.
Bosch Master, ASE Master, L1
33 years MB technician