gear question

i am in the process of changing gears and i need to find out what the original setup is because mine is not right. things i need to know
1. why are there shims just floating in between the two bearings on the pinion?
2. also should i start off with the original shims and use that as a starting point and adjust or is there an easier way to figure it out (calculation, etc...)
3.

If you need to ask these questions, you may be in over your head.

If the shims you're talking about were under the outer pinion bearing, they are for preload. If they were under the inner bearing, they are for pinion depth.

Yes, the original shim packs are a good place to start...