Checking compression

I think where the "20" comes in is the readings on a dynamic check should not vary more than 20psi across the board, and on a static (differential) check, you should have less than 25% leakdown.
EG...175 on 7 cylinders and 105 on 1 = bad.
On a leakdown check, 175 (minus 25%) = 132 (minimum allowable)
Of course, both of these refer to warm engines that have been run and oiled, as was mentioned.
It's also important to not reject a cylinder based on one compression test reading.....