is there a service out there from a lab that you can send a sample of a batch and they give you a certified result for what your aBV is after adding sugar. This redistilling in a lab still is getting old.
I suspect you are going to get even more sick of paying lab fees, when you see what a typical lab is going to charge you.
Work in weight, not liquid volumes, everything by weight, get good scales.
Do your gauging of your unblended source alcohol.
Blend your water, sugar, flavor, by weight - be accurate, neat, no losses, no spillage.
The end result, proof, will be accurate, every time.
Your sanity cross-check is bottle count vs expected.
You can argue that there might be unaccounted error due to process variation, but I'll argue you aren't accurate if you are holding a batch in an open tank for a week while you wait for lab results (unequal evaporation).
i do everything by weight and not by volume, for one it takes the temperature out of the equation. Your argument on evaporation is also very valid. i just want a check and balance to my recipe. Sugar is a pain in the ass to work with. i use alcodens now for creating my recipe and it gets me dialed in but would like to see how close. i just figured a lab with a dma 4500 could quickly test and email me the results. i just dont want to drop $45K on my own alcolyzeR