I don't understand why I should mix fuel by specific gravity. What is wrong with mixing by volume? The only reason that I can think of to use a hydrometer would be if I suspect that my nitromethane has been diluted and is not the advertised purity.
Almost all if not all fuel companies mix by volume. You could consider it an industry standard if you wanted to. Even if there is a 1-5% difference in what the advertised content is, that is not going to be of much hindrance on the performance of the engine. If you get your nitromethane from a reputable source, they aren't going to sell you 95% nitromethane and say its 100%. If you distrust the supplier, then find another supplier. In now way is it necessary to get that technical unless you're paranoid about having the mixture ratios being off a few percentage points.
A model engine isn't going to care if you feed it 12% nitro instead of 15%. The only noticeable difference will be the tuning.
moe7404 - you (and the rest of us) have beat this horse so badly its not only dead, but all of its limbs and head have fallen off. Just agree to disagree and move on. You mix your fuel your way and we'll mix it our way. OR if you question it that badly, why not call a few fuel companies and ask them what method they use to mix their fuel. Better yet, buy a gallon of 2 or 3 different brands of fuel and get out your hydrometer and test the nitro content. While you're at it, verify the methanol and oil content as well. I would be curious to see if 20% nitro is really 20% or if its 19% or 21%. I'm dying to know. (not really)