car4jim, I think the directions are one once for every 2 ˝ gallons or four ounces for ten gallons. I checked out in the garage and do not have any cans out there to be sure but the directions are on every can when you buy one. I just use an old paint measuring cup or just a plane six once drinking cup that can easily be bent to fit the gas tank nozzle and fill it about two thirds the way up, maybe a little more. “If a little will do good, a lot will do better.”
Never felt that you need to be any more accurate than that.
Rick