Is this a viable method?: In theory you can calculate an estimated ABV by using the formula 17*X%*YL. This means that 17 gr sugar produces 1% alc. in 1 liter mash. So if you know at what alc.% your yeast gives in and how many liters mash you make you can derive how much sugar you should add to reach there.
To know for sure when all sugar is consumed by the yeast, using a hydrometer and the formula that 2,7 gram sugar is 1° Oechsle in 1 liter mash, you would be able to know when all sugar is gone.
If you keep track of consumed sugar on the hydrometer scale, and when it finishes bubbling as predicted - would you be able to verify the ABV with a refractomer measuring plain % alcohol??