That's impossible to say with accuracy. The only sure way is to put your new gaskets on, torque 'em down, then remove and measure. Every brand will be different, even whether you're torqueing to 45lbs as Ol Ron recommends, or 55lbs as most books recommend will make a difference. Also remember when you mill the heads, you will probably have to re-dome the chambers, assuming you're running domed pistons. Please don't ask me how I know this.....Mostly a question for Ron, but anyone chime in.
I'm looking for the compressed thickness of a fiber head gasket, not a copper one. Anyone have that info so I can do the squish test to find out how much to mill off?
Just removed a set of Felpro's measured .057"