If I understood correctly. You are asking how to calculate a total ABV if you add more fermentables in the middle, right?
I would take an original, and final before racking. Calculate the difference in gravity points (i.e. 1050-1010 = 0040) Then rack onto your sugar, stir, take another sample and maybe you are at 1020 now. Take you final gravity and say you are back at 1010. So your total sugars would now be 0050 and you could multiply that by .131 to get you ABV. The problem here is that you may not have an even dilution of that sugar. So I would go with taking the normal gravities, then weighing out the amout of sugar and using the fermentable ppg number to figure out how many gravity points it is for your volume (5 gallons).
To calculate the sugars you can use this table to see the ppg http://www.byo.com/resources/grains?gtype=5
So for example if you had 1 pound of dextrose in five gallons you would take 1037/5 or really 37/5 which gives you a little over 7 gravity points for your volume. So that should be the increase that you could add to your OG-FG (before racking) differential.
Not sure if this all made sense, but ask questions if it doesnt.