I agree with all of the people who say you are wasting your time, but it's your time so here are some rough calculations . . .

The easiest way to figure it out is to figure out how much corn sugar you would add. If you're adding 3/4 cup, that's around 5 oz by weight. Corn sugar gives 46 points per pound per gallon, so you're adding 5/16*46/5 = 2.875 points per gallon. If you are really that rigorous, monitor your beers well, and brew regularly enough to know exactly where the beer will finish, then you can just keg when your beer is 3 points above the FG and let it carbonate. Some experimentation will let you dial it in.

If that's too risky for you, then just save some of your batch in the fridge or freezer until it is done fermenting. Once you know the OG and FG you can calculate the RDF to calculate how much of the unferemented wort to add to the beer at kegging, it's pretty straightforward algebra to get you close. This formula will get you in the ballpark, where %Sc = % sugar (current), %A = ABV, and %So = %sugar (original).

%Sc =(1.4*%So - 0.21*%A - 1)/0.4

You use the current %Sc to calculate the RDF . . .

RDF = (%So-%Sc)/%So

Once you know your RDF you can figure out how much to add. If your RDF is 55%, which is not unreasonable, and the OG was 1.060, you can assume you're getting 33 points from each gallon of wort. You need 2.875 points per gallon, so . . .

33* Va + 0*Vo = 2.875*(Vo+Va)

Where Va = added volume and Vo = original volume

Solve for Va . . . . Va ~= .1 * Vo

So you should add about 1/2 gallon of your unfermented beer to 5 gallons to carbonate it.

**I don't do this.** I hope this helps - the theory is sound, but I may have muffed the math along the way . . .