It's not 8.4:1. That's the whole reason that .100" is pretty much the starting point for head milling. But it falls on deaf ears like a lot of other advice. These engines blueprinted down in the 7s for compression. They were rated at 8.4, but they always measure way shy. What part of "you're going to have to measure" do you not understand? We cannot tell you what your compression will be because we don't know what it is now. And you cannot use 8.4 as a starting point, because that ain't what it is either. I've never understood guys milling any less than .100" off because the cost is the same. .100" will get you "somewhere" in the 9:1 area. But again, YOU NEED TO MEASURE what you have now and CALCULATE what the difference will be after the .100" cut. There is no magic wand to wave to tell you the answers WITHOUT MEASURING. No matter how many times you ask the same question differently the answer is YOU HAVE TO MEASURE. It's grammar school math. It's not hard, but you're sure as heck making it that way.