Ceck my assumptions on Leak down test please.
 

I bought a cheap leak down gauge on Ebay.

When I hook air up to it ( not hooked to car) and adjust the regulator (in put gauge) to 50 psi the second gauge (leak gauge)reads 40 psi. Very linear relationship , lags by 10 lbs all the way up to 80 or so (as hi as I tested it). No leaks in the assembly (that I can hear). At lower pressures its not so easy to read.

So I plug it into the motor. and on the first cylinder it shows 38 PSI on the second gauge (output), 50 PSI on the regulator gauge (input).

So 4 % leak (on this gauge). I figure the second gauge reads 10 pounds low and its leaking 2 psi at 50 psi so at 100 it would be 4 psi or 4%

Any problem with my logic?

I realize its not a definitive number with the low quality of the gauge. But I think I am safe to say the cylinder is less than 10 % leak which is OK for a high mileage motor??