1.28 ounces is 1% of a gallon (128ozs). I use Power Master which is 18% oil, 1/3 castor - 2/3 synth. I add four ounces of castor to every new gallon which brings the total up to approximately 21% oil.

It doesn't have to be exact. When you add four ounces of castor to a gallon you now have 132ozs of fuel, so it's off a little. You could always remove four ounces from the new gallon and replace it with the castor if you really want to get precise, but I'm not that nitpicky.

David