compression vs. compression ratio

what i've been told:
72 RR 400, 452 heads, .509 cam

what i know:
72 400 block, pistons have a pentastar and casting number on the inside of the skirt, so they MUST be stock, i assume, .509 cam IS true, 452 heads

what I don't know:
milling amount (if any), gasket, and current cranking compression (don't have a tester yet

so...looks like i'll have to tear it down eventually, but since it runs and there is no reason for me to go that far into it, i suppose i'll have to wait until i have to take a head off....