you could probably back calculate it off of the Beersmith carbonation calculator for your recipe, just switch corn sugar to malt extract and set your volumes of carbonation.
Here's my real life example
5 gal on Wit at 2.7 volumes CO2 needs 170gm of DME (454grams per pound)
My OG of the wort is 1.050, or 50 points per gallon.
170gm/454 gm = 0.374pounds of DME
DME has 44 points per pound
SO, the calculation is
0.374 pounds x (44points per pound/50 points per gallon) = .33 gallons of my wort
If I can recover this from the pot, pressure can it with maybe 10-15 min at 15 psi and it'll be ready to go at bottling time. The bonus is that it has the same hop or spice character as your beer
Hope this helps