Generally speaking, .030" is the max machining on most passenger car and light truck drums, regardless of overall diameter, with .060" being the discard dimension. Usually, this is either cast or stamped on the drum, though not always. Unfortunatly, many "brake mechanics" have a tendacy to cut drums until they are true, regardles of how much has to come out to do it. Couple that on top of the fact that many shops brake lathes are in dire need of having the arbor zeroed out, that can take quite a bit of material out to true things up. Personally, if I have a drum that's taking more than .025" to clean up, I replace them. Same goes for rotors, depending on what the min spec is.
