You need to use 33 parts gasoline to 1 part oil.
A gallon is 128 ounces. Divide this by 33 to get 3.9. So for each gallon of gas, add 3.9 ounces of oil.
I put 4 ounces of oil in the tank and add a gallon of gas, which gives me a ratio of 32:1. Close enough.
If you use 3 ounces of oil to a gallon of gas, you'll have a ratio of 128/3 = 43:1.