I use a leak down test almost every single day at my job on internal combustion engines, maybe more like once every two to three days. Either way my point is that I have alot of experience with using the leak down tester as a diagnostic tool. I almost think that this tread is people over thinking about it. I can promise you that if you have a any valve issues or ring issues you will know it! if it is over 20% there is a problem somewhere period. i have seen cars with 15% leakage with almost 200,000 miles on them and still running fine considering the wear on cylinder walls and rings, so i believe that a fresh motor with even too much ring gap wont even hit 10% on an up too temperature engine, would almost put money its not even to 5%. of course, i dont use cheap tools not that they dont work will cause i dont know that, its just when it comes to any measurement type tool I only use will known name brand tools. your engine will most likely have more than normal crankcase pressure so you may end up need to use a catch can on the crankcase vent. burnt exhaust valves will usually put the percentage around 30-40% and bent valve or damaged seat will be 80% or more. for those that are interested, OTC is the company that makes most of they tools similar to leak down testers for other tool companies like matco, mac, cornwell, and so on. so if you buy OTC brand you cant go wrong.
And the comment about the not tearing into unless its more than 20% i would agree with ONLY if you can hear or feel the are coming from the crankcase breath hole, if its blowing out that exhaust or intake then tear it down and dont spray it but i would put money that if you got a valve seal issue its going to be well over 20%. this much past the rings will only make the engine down on power and excessive crankcase pressure.