Yes very true, your request is much more common.
What i would like is a formula that lets me take a density reading and determine that i have X proof alcohol. Then i would like to add an exact amount of water then take a new density reading showing a lower proof because of the water i added. Then i would love to be able to have something that says i have an unknown volume of X proof that i add a known volume of water to and also have a final proof. So by using the change in proof and the known quality of water added determine my total volume of spirit at the final proof.
Does that make sense?