Nitro Methane, the "nitro" in comercial model aircraft glow-plug engines is an oxidizer. For a given compression ratio, nitro induces combustion earlier like an advanced set of points in an ignition engine. Early combustion creates more power. The engine gets more tempermental and NV settings more critical.
10% nitro means that 10% of the fuel is Nitro Methane, commonly known as a rather cool fuel. 30% means that there is 30% by volume of the fuel is Nitro Methane. Some have worked with nitro ethane but that generally is more difficult and sensitive than Nmethane. Once one gets beyond 40% Nitro Methane, operation does become rather sensitive. I have pylon-raced at 70% nitro, 10% methanol, and 20% oil. Not easy as the needle is set to blow the glow-plug at about the last half of the 10Th (final )lap. Didn't always work!. [&o]

Not a function of the engine displacement. Nitro should be based on the timing of the engine, compression ratio, and the job for which it is to be used. In the US, 50% + nitro may well be used to attain speed where as in Europe, and nitro not being allowed in speed events, the use of 25:1 and higher compression ratios get the job done for speed. Using sport props in compression ratios of 8:1 to 12:1 or about, nitro above 20% is, IMO, a waste of time and effort.
If one fuel was always the best, then there would only be ONE fuel.

As someone above stated, whatever you like. Some think that higher price mens greater fuel performance. I prefer Wild Cat because the mix WAS always the same as quoted on the label. Now that a certain distributor has control of Wild Cat, it seems to be like others. The one thing I do look for is fuel with at least 18% oil which is 80% synthetic and 20% Castor Oil.
If I were still into racing, then that would require different mixes.

I have a source where I purchase a number of cases usually once a year. Check it out. I pass through the area once a year back and forth. I load up the truck.