In the end, if you know the total quantity of fermentables and the total volume, you should be able to take a initial and final gravity reading to determine remaining sugars and have an accurate abv, right? Obviously as you add sugar, you are adding volume which will change your gravity and your abv - which is why it is crucial to know the total volume and sugar content.
As an example, I make mead - so I know that 40lbs of honey in 12 gallons of final must will produce around 18% if fully fermented. My OG would be around 1.132. If I actually start with 30lbs in 10 gallons, my OG is around 1.120.
I use the 2/3rds rule of thumb, and at a gravity of ~1.080 (2/3rds of 1.120) I add 5lbs of honey in 1 gallon of must, bringing my total honey to 35lbs and total volume to 11 gallons. Then I do the same one more time again when the gravity has 2/3rds its reading after the addition.
Now, I haven't kept good enough records on gravity readings at these steps in the past, so I am going to do that with a batch I just started a few days ago. I would like to have the equations nailed down and in my spreadsheet for calculating when to add honey and what the gravity bump should result in....