Keep in mind, some fuel makers blend by weight and not volume. Volume is the accepted practice for most, which gives you the better bang for the buck over weight. The specific gravity of a given fluid changes with the air temperature so today it might be 13%, tomorrow it could be 12%.

RTR companies are aiming their products at new folks. Basically making everything as idiot proof as possible.

As as to mixing your own fuel.. Get a scale that goes up to about 12#. We have a digital kitchen scale I use. Figure out the amounts by volume you want in a given mixture. Now calculate those volumes as a weight. Once you have the weights of the individual chemicals sorted, put empty jug on scale. Add methanol to the calculated weight, and zero the scale. Add oil to the calculated weight, zero the scale. Do the same for the nitro. Add a few drops of your favorite food coloring and shake. I convert to mL as most specific gratifies are in metric - g/mL. Nitro is approx. 1.16 g/mL, methanol is .71 g/mL, and castor and Klotz synthetic are around .97-.99
g/mL.