RE: Nitro Percentage
Determining the compression ratio will give you a good indication of how much nitro you can use without causing detonation, that is if you really want to get technical. I do agree with Neal in respect to some engines will respond much better to more nitro, namely going from 20% to 30% being a noticeable increase. I've seen engines with trapped CR anywhere from 8:1 to 12:1. The engines running 10-12:1 will run well on low nitro fuel (0-10%) and may detonate on more than 10% whereas an engine with a CR of 8-10:1 will probably run 30-35% fine. I think even beyond 35% at 8:1 CR you will still need to shim the head a little bit and/or run a cold plug.
Lowering the head (taking out shims) will increase power but will reduce longevity to a point. It can also limit the amount of nitro you can use. In a lot of cases, the shim is only installed to control the CR and not to seal the head/head button to the cylinder liner. One can use a 4-stroke glow plug to increase the compression ratio as well as those have a projected tip that extends into the combustion chamber further. I've done experiments using a 4-cycle glow plug and removing the head shim and getting really reasonable performance running without any nitro at all. (Yeah, I'll go outside my comfort zone from time to time and run zero nitro.)
For the average modeler, I'd just keep adding nitro until you blow plugs. Once you blow a plug or two, either shim the head .1-.2mm, run a colder plug, or drop the nitro content a little.