If you can accurately measure the head thickness from the upper head bolt boss to the underside, then the compression ratio can be calculated quite accurately enough for most purposes. Volvo varied CR by head gasket thickness, and cylinder head thickness. On the early B20E engines the "lip" of the head outboard of the spark plugs is noticeably thinner. Measure the head thickness with a dial caliper to within a couple of thou and post the result. I'll be able to give you a good ballpark idea of the CR.
Back in the 1980's and early 90's I converted many B18 and B20 engines to the thicker head gaskets to reduce CR and allow stock ignition timing to be used with regular fuels here in Canada. In every case the thicker head gasket did what it was supposed to - lower CR by about .6, and allow normal ignition advance to be used. Pinging and run-on were eliminated. The gaskets were so much thicker that the water pump o-rings had to be 50% taller to allow a good seal. These parts were supplied, and recommended by, Volvo themselves.
I am aware of the technical articles written by some famous engine builders, and the arguments for or against the effectiveness of "squish, quench, pocket gases" and so on. In Volvo engines the thicker head gaskets work well in my experience.

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.

What will really help is if you give us both the thickness and
the depth of the combustion chambers. From that I could probably
give a fairly close estimate based on manual data, and also probably
tell you if it has really been milled or if it is just a thinner head.
--
George Downs, The "original" Walrus3, Bartlesville, Oklahoma