Pinion depth is usually marked on the gear, it's specific to the gear set you're installing. Gear sets are usually matched for this very reason. It's the relationship of the pinion hear to the case. You have to measure it with a pinion depth gauge.
If you took you diff apart, there should have been shims between the pinion and pinion bearing. These are what's used to adjust the measurement either further down in the case away from the ring gear or further up in the case closer to the ring gear. If you're re-installing the same gears, keep the shims and you should be in the ball park.
