Compression ratio is the relationship between the "swept volume" (ie bore area x stroke) and what is often termed the "squish volume" (which is the volume left in the top of the cylinder when the piston is at top dead center).
compresssion ratio = squish/(squish + swept)
Domed pistons and skimmed heads reduce the volume of squish and therefore raise the compression ratio.
Compression pressure should be equal to air inlet pressure x compression ratio (+/-gains/losses).
ie. for an engine running at atmospheric pressure (14.7 PSIA) and a compression ratio of say 7.5:1 you should read 14.7 x 7.5 = 110.25 PSIG (+/-).
Of course this is all subject to valve timing, pulsing in the inlet, friction, altitude and many other factors but that's the basics.
Getting a good baseline reading is probably a good idea and may be useful for troubleshooting in the future. Trouble is to get a good reading the engine should be at normal operating temperature, but when you're troubleshooting it's generally because the thing ain't running.