Uniformity of compression pressure between cylinders is far more important than the actual pressure reading. If you use three different gauges, you're likely to get different readings on all of them. I usually plug my compression gauge into the shop air line to get an idea of how accurate it is before starting a test.
A 7:1 compression ratio at atmospheric pressure of 14.7 PSI per atmosphere would be 103 PSI under ideal conditions (no leakage, and a very accurate gauge) That's not likely to happen, so a reading of 100 on the compression gauge is going to indicate somewhere around an 8:1 ratio with a little piston ring wear and/or an inaccurate gauge. Early stovebolts don't come anywhere close to an 8:1 compression ratio, so compression in the 80-90 PSI range should be close to normal for a 216, slightly higher for a late model 235 with an 848 head.
Jerry
