Need Advice on Machining the Intake Surface of the Head (gasket crush)

It would have been best if these measurements had been taken before the milling as well.
You don't know if the machine-shop created this skewed plane, or if it was there in the casting from factory.
It's also difficult to tell if the heads have been milled before.

Having worked with a Van Norman head-surfacer a couple of times for instance, I know it can be a hassle sometimes to properly clamp down heads that only have angled surfaces on the sides.
It's easy to mill at a veryslight angle if you're not careful mounting the head.