I use unfermented wort to carbonate my beer. To determine how much wort to use, I use the following formula (forgive me, I'm going off of memory here):

(V X 12) / (OG-1 X 1000) = Qts of wort (gyle)

V= Volume in Gallons

So, if you have a 5 gallon batch and a 1.060 beer, you will save 1 qt for carbonation.

As long as I'm accurate with my volume, it comes out ok, yet fairly inconsistent. It does seem that the higher the gravity, the lower the end carbonation is. Also, the lower the gravity, the higher the carbonation. The closer I get to 1.060, the more consistent the carbonation. Does anyone have a better formula?