Checking compression

Okay...it's been a LONG time since I've checked compression...let's see if I remember correctly...
let the compression gauge needle jump 4x's

I remember something about '20'...is it all cylinders within 20#'s of each other, or 20% of each other, or does '20' not factor in at all?

also...how will the compression differ from a cold engine to a hot engine?