Under 20% is considered acceptable. Frankly, I use this more to diagnose a problem, finding out why a motor isn't running right. Racers like to do these looking for any little edge they can find. If your motor isn't an all out max effort race motor, and you're just monitoring wear, this kind of searching for trouble can drive you crazy.
If you rock the crank back and forth a couple degrees while doing a leak down you can change the readings quite a bit. What I mean is, at TDC you may have 20% + leakage, but you can rock the crank a couple degrees one way or the other and all of a sudden your leakage drops to 6%. So is the cylinder bad? Do you maybe have a valve that's worn a little and not sealing quite as well as it used to? Maybe a little carbon build up. Is there a scratch in the cylinder? See, this can make you go crazy.
Monitoring how the motor starts, idles, accelerates will tell you a bunch about your motor condition. Watching your spark plugs for things like oil consumption or excessive unburned fuel will tell you you're getting close to maintenance time. hope this helps.
