Here is how I calculate fuel to oil. It will work with any ratios of gas/oil or amount of fuel. All you have to do is convert gallons to ounces or litres to millilitres . You have to compare apples to apples. Don't worry about understanding how this method works just know that it does.

I forgot a zero on the metric 20:1 amount. I have 5ml I meant 50ml. That would be disastrous. Using the same method just moving the x would be 200:1