The Min Thickness is stamped on the rotor if they're MB rotors. Usually if it's above the service spec you just reuse the rotors, I've never machined an MB rotor. The service spec is usually about .5 mm thicker than the Min Spec stamped on the rotor.
If you can't locate the Min Spec on the rotor, let me know which of your cars you're working on and I'll see if I can find the specs.

Gilly