I've been researching offsets to get a better understanding of the concept as a whole.

When it comes to the e60, I understand the base offset (I believe) of the 18s are ET20 (please correct if wrong).

From there, as the rim size increases, the offset needs to decrease.

If you are running 19x9.5, it seems like an ET18 would be necessary for the front rims.


The issue I don't understand is spacers. If you add a 15mm spacer to a ET30, you get around an ET15 (I'm guessing).
But how large can you actually go to fit in an e60 wheel well?

 

20mm is the max spacer size you wanna run with out putting to much strain on the hubs. For the front yes the offset needs to push the rim away from the strut. On the rears you can either push towards the fender or pull them towards the suspension for that "tuck" look. Once you lower the car the fender lip comes into play

 

Ok, that makes sense.