John is quite right that one or two companies short change you on both oil and nitro by mixing using weight. Mixing by weight is a very good idea IF it then approximates very closely the accepted values by volume. Unfortunately, evaporation tests don't seem to show this (for oil content) so you can imagine how much nitro is really in the fuel
Mixing your own fuel should always be done by volume, for one thing it's far simpler and the coefficient of expansion is so small that over any reasonable temperature range you wouldn't notice any difference.