Compression reading by itself is only a relative reference mainly because of the factors you mention.

You would have to make as many factors the same as each other and compare too engines (A new one and a old one of the same type) for it to have a concrete comparison.

People that mess with them all the time develop a pretty good feel for an engines worth by putting a little oil in the cylinder and pulling it through compression several times. I try it fast and slow so I can feel the "leak - down" as well as the over all compression. While I'm at it, I feel for the piston - rod fit as well as the main bearings. I don't bother with the compression gauge itself.

Good luck,

Jim