cylinder head milling questions(before i go to machjine shop)

I want to take my rtc6 head down to have it milled to increase the compression ratio. I want as much taken off as possible without having to shorten lifters or pushrods. Im hoping somebody might be able to tell me how much i should take off?
When the piston is at top dead center, you need to see how many thousandths of clearance there is between the top of the piston and the deck of the block
Knowing that in conjunction with the size (cc's) of your combustion chamber will give a better picture of what compression ratio you are at now, and you can use that information to determine what will be needed to reach a certain compression ratio