As an aircraft mechanic I've done lots of these, the publication AC 43.13-1b outlines the correct procedure. The test is done on a warm engine and, to start the test, air is introduced into the cylinder while the piston approaches TDC and is finalized with the piston at TDC. The airplane has an advantage because of the prop, you can grab a prop blade and manipulate the piston position instead of just parking it at TDC. This assures that the ring is fully seated against to bottom of the ring land.
and to add to what John said, there are different orifice sizes based on bore size. Put in 80 PSI and you should read better than 60 for some engines, 42-43 psi for others as long as it is not through the exhaust.
