Conversions are really unnecessary if you follow the example below.

As an example, if you make up a fuel containing 18% oil, 15% nitromethane and the rest (67%) methanol and you want to make up 3000 ml of it, you use:

0.18 x 3000 ml of oil, or 540 ml of oil in this example

0.67 x 3000 ml of methanol, or 2010 ml of methanol this example

mix the oil and the methanol

0.15 x 3000 ml of nitromethane, or 450 ml in this example

add nitromethane, remix well and pour up and tightly cap your bottle, store at room temp in the dark if possible.

You can do this with any fuel you want to make, all the ingredients are added in simple volumes, based on the final volume that you want to make up. Hope this makes sense to you.
It IS easier to use metric, but we got used to a length based on the length of the foot of a king, so what do you expect? :-) We have POUNDS instead of nice grams and kilograms. So even into the 21st century, we keep inching along. Pun intended. Enjoy your mixing, it seems to be one of the very few ways we can actually know any more what is in our fuel!