You have a 7.9 gallon primary? If so, there's plenty of room to simply add half the F-Pack.
Take an SG reading before you start, add the pack, stir well, and take another SG reading. This will enable you to figure out how much you bumped the ABV.
Are you using a 23 liter / 6 US gallon carboy? 23 liter kits are 23 liters at the beginning, e.g., the kit reconstitutes to 23 liters. You have noticed that you have less after the first racking. With the F-Pack the carboy will be closer to full, so watch when racking into it.
If the wine turns out too dry, you can add sugar to taste after stabilizing.