I think the FAA will accept up to 60% leakage and still let you operate the airplane with the proper sign offs
The procedure is different also on aircraft leak down test, you don't test it at TDC, you pull the piston up being tested with the prop to about 120 to 90 degrees BTDC on the compression stroke and have someone read the gauge while holding the prop at that spot
Sometines I will pull a piston up on one of my motors until the valves close to see what the bottom ring sealing looks like