Magnet isn't strong enough to pull through the aluminum wheel. I haven't needed to make any adjustments on my MB. For a quick check I've used the register for the center cap realizing it is just a quick alignment sanity check.

For my oval track car the magnet slips over the grease cap and engages the steel hub. For my Formula Ford, a caliper piston parallel ground gets me over the grease cap in the front or rear hub nut and in contact with the flat surface of the aluminum wheel.

I've used different caliper pistons for other cars, at some point I need to modify some old alignment wheel adapters to accept a flat plate so I'm not having to rig up adapters.

Longacre sells threaded adapters for common race car spindles, something similar could be rigged up for a MB.