Hi JumpyMan
1QwkSport2 has given some good advice on using 'a scale' to measure your fuel mix (due to the difference in specific gravity of each fluid).
Its much more accurate than measuring by fluid graduations (ie: mixing by by pouring into a graduated beaker/ jug and reading off the graduated markings by sight).
I find mixing in 1000ml batches (in this case grams) convenient and then tipping into a 5Ltr can after each mix - 'purely for ease and less mess'.
Across the pond you'll probably be using fl oz. But same principle !!
NOTE: DO NOT CONFUSE FLUID OUNCE (FL OZ) WITH OUNCE. THEY ARE DIFFERENT UNITS. FL OZ IS SPECIFIC TO VOLUME (LIQUIDS).
(1000ml = 33.82 fl oz)

Enjoy....!