I've been using the Ratio Rite cup for 40yrs between motorcycles, yard equipment, and my model engines. I could do the math, but why? Seriously there's better things to ponder then ratios isn't there?

Spend the $6 and get the cup, find the ratio you're wanting(it's all molded into the plastic on the cup), find the column for the amount of gas you want, put the oil in the cup to the line of the ratio, pour oil in the gas can, run down to the gas station, order up the gas. If you measured/poured oil for 1 gallon, just put 1 gallon of gas in the gas can. It's so so easy. It can't be any easier.

I don't mind math at all, and I'm pretty good at it, but there's more chance I'd make a mistake then there is if I just use the cup. My engines are worth buying a $6 cup. Good luck.