You have to calculate the algebraic percentage of alcohol in the finished product.
You begin with 25.4 ounces of 100 proof (50% ABV) vodka. When you finish, measure the total volume. Divide 25.4 by the total volume and multiply by 100 to get the final proof.
But are you assuming that the alcohol and water of the spirit get absorbed into the solid matter at an identical rate? Because it's been my experience that the solids (fruit, herbs, etc.) absorb more alcohol than water. If you taste a piece of fruit that's been macerating in booze for some time, it hardly tastes like fruit, just fibrous booze. And the resultant liquid is much more flavorful and has a lower proof than the spirit used, hence the ability to freeze. Without a hydrometer, how can you tell what the proof is?
I never assumed that precision was important to this equation. If you have to be that precise, a $10 hydrometer is the obvious solution, and probably faster than a caculator.