That's actually the way it's always done but it's also where it causes confusion. Say you had a fuel with no oil and was 50/50 nitro/methanol, it'd be called 50% nitro. I'll work in litres here because it's soooo much easier . Take 800ml of that fuel, that's a mix of 400ml of nitro and 400ml of methanol. Now add 200ml of oil and you now have 1 litre of fuel. The oil content of that total fuel is 20% (200ml in 1 litre) and the nitro content of that fuel would appear to be 40% (400ml in 1 litre) so it'd be marketed as 40% nitro. The remaining 40% is the methanol. But 40% nitro and 40% methanol still leaves an equal amount each of the fuel part of the mix. No matter how much oil you add to the blend then the nitro and methanol always remain the same 50/50.