There are quite a number of different castings for B20 heads -- I don't have the numbers handy, but as they've likely been modified anyway, that wouldn't tell you much in any case.

The ONLY way to know what the CR is is to do some basic measuring and some basic math. I've heard "rule of thumb" theories that don't even come close; worse than useless.

Tell me the volume of the combustion chambers, how far the piston is below (or above) the deck of the block at TDC, and the height of the gasket after crushing (each gasket has a before and after crush number), and I'll tell you what the CR is (I already know you're .040 over and at stock stroke).

Without those numbers, you will never even have a rough guess as to compression.

Now, please please please do not use a thicker-than-stock head gasket in an attempt to reduce compression. It will reduce the CR and performance/economy along with it, but it will make pinging WORSE. I know of what I speak here, really truly. This is not just theory - there is a science to how this works.