Engine milling maths

The closed chambers will require more to get a cc off than open chambers....IIRC, you will need to mill off about .006" per cc, or .030" total for 5 cc's. If you take off a bit over 5 cc's, a touch with the grinder in the chambers will get it back down.

I would not count on all machine shops to know this... so good that you are checking.