The way I've found easiest is to figure what volume is NOT oil and go from there.

For a first example let's figure a gallon that is 17% oil, and we want it to be 25% at the end.

17% oil means 83% alcohol and whatever, so 0.83 times 128 (gallon) gives us 106.24 ounces of fuel. Wanting this to be 75% of the final mix divide the 106.24 by 0.75, and we come up with 141.6533 ounces total. Subtract the 128 ounces we started with leaves 13.6533 ounces of oil to add, ending with 25% oil content.

Your quart of 17% has 26.56 ounces of "Fuel," that will be 82% of 32.39 ounces of 18% oil, so add 0.39 ounces of oil to get to 18%.

For 19% oil add 0.79 ounces of oil to a quart of the original 17% mix.

Finally, if you want to go from 17% to 20% oil add 1.2 ounces oil per quart of the original mix.

Bill.