Another option that I would guess would work, is to determine just how far you are lowered from stock in the rear. That number is easy to find if you have lowering springs as it should be a published value, less so if you have adjustable suspension. If you've got coilovers, there are published values from BMW for stock ride height from the ground to the wheel arch (or something like that) that you can compare to. If I'm thinking correctly, measure down from the center of the hub by the amount you are lower from stock ,and then up 8mm from that point to set the preload relative to your ride height.
As far as the vibration, unless your alignment is really out of whack, it is most likely that the new RTAB is 'uncovering' and issue elsewhere by transmitting vibrations that were previously being absorbed by the worn bushing.
note to my idiot self. it usually helps with vibrations to actually tighten lug nuts.............
rtabs all better now.


