I hate to cause you to doubt your future machinist, but in my experience more are good tool operators than good engineers. Meaning: you should be the general contractor if you want good results and give him very specific instruction:
1. the original error.
2. the exact angle or extra thickness at deep end to modify.
3. "please record what this was" i.e., "Took .036" off at 1.5°.
4. the intake seats at the deep end (largest mill) will probably need to be done again, and this means either:
a. the stem heights will vary, or
b. some seats will be sunk to match them
You won't know until you see it after the mill, so he should call you at this point.
5. the "hook channel" (gasket surface relief from the intake seat to the exhaust pocket) will be/has been removed by milling. IMHO this should be restored, but this is delicate hand work unless you want to pay him to make a bolt-on guide for a vertical mill cut.
