Cylinder head machining advice.

[Smooth_One]

Hey all, towards the winter time I am going to pull the head off my A and probably mill it as minimal as I can just to get it flat for a new gasket. I pulled the center not loose with the clamp that holds the armored cable to it to replace the cable and the stud came out with it unfortunately. I ignorantly thought I would just put it back in and tighten it to spec but as soon as I walked away from the car for about an hour coolant was coming up through the stud threads around the nut. And then I ran the car for a few minutes and had some coolant in the oil so I drained it immediately and put fresh stuff in it.
So after all that reading if you made it this far… My question is I don’t know if my head has ever been machine before but I will measure it before I do any machining.
What is the minimum thickness I can get away with on the head before I have to clearance the valves?
And in your opinion what should be done to the actual top of the block surface before reinstalling the new gasket besides rigorously cleaning the surface?