Your technique can matter. To get an accurate reading, the compression gage must displace the same volume as whatever component you removed (injectors or glow plugs). My Harbor Freight tester has a ring you must use on the "injector simulator" to fill the volume. It also has a tubular hole that seems different than the real injectors. I have had mixed results from leaving the injector heat shields - last time the shield increased the reading a bit. Seems smarter to use them. The volume that matters is up to the check valve in your tester, and that Shraeder valve must be a special type with very weak spring (usually a white collar), not one for a bike tire, auto, or AC port. You cannot calculate expected psi from the compression ratio, because the camshaft makes a big difference. Different cams can cause a x2 difference in gasoline engines, especially "racing cams" with high intake-exhaust overlap. Fortunately, we have little choice in camshafts. Cylinder pressure also varies w/ speed (organ pipe dynamics in the intake), so the starter spinning speed could make a difference though that is more an effect at much higher rpm's. Perhaps a greater starter effect is how well the oil seals on the rings - faster stroking leaves less time for leakage. I expect that the thicker oil in a cold engine would increase piston sealing, and also give less thermal loss to the cylinder walls (as does faster speed). In my tests, it takes ~12 compression cycles to reach max pressure on the gage. In gas engines, people say it should come up in ~3 cycles (usually does for me), but I suspect that has more to do with the design of the tester, i.e. how much volume between the check valve and gage.

I recall measuring ~250 psig in most cylinders when my 85 300D turbo was getting hard to start on 35 F mornings. It failed a year or so later. The 82 replacement engine (craigslist) measured ~410 psig in all cylinders (cam looks new) and my 84 engine (130K miles) measures ~405 psig in all except one 350 psig one (will check valve gaps). These are all with a warm engine (hot when I started removing injectors).

BTW, while your injectors are out is a great time to adjust your valves. It is so easy with no metal tubes in the way. I once tried using the glow plug holes for compression, but seemed harder. Since I made my pop tester, it is a good time to check the injectors anyway.