You will need a starting point. Because of the domed bottom the first few gallons are off the mark. What I did is to start at 5.5g and 1g increments up from there. I wish a had an metal engraver, I had to use a sharpie.
OK assuming the volume of boiling water is 4% larger than that of cold water. 1.144 (2.90 cm) inch instead of 1.1. Honestly the sharpie marks about 1/16" (.0625) wide. I don't think that small of an amount is going to make much of a difference. (I don't think my marks are that accurate).