if you remove a total of .030 with milling and thinner head gasket is that the amount you flycut or is there a ratio to follow?

 

No.
You measure how much clearance you don't have and cut that amount plus the recommended ptv clearance which is usually .080+
Personally I cut with no head gasket unless I'm worried about losing compression or cutting the piston too thin.

The only common number you use is finished ptv clearance. Everything else has to be measured. IMO if you have 20 engines lined up in a row all identical, you still have to measure each one.