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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.
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