I am using plastic single gang boxes for my receptacles. I am also using 12 guage cable. I have read that you can't put more than two 12 guage cables in a box. The boxes are 22.5 cu inches I believe. I think I recall reading that you can run two 12 guage cables and one 14 guage cable.
The reason I ask, is because I want to tap off of a couple of receptacles for outdoor receptacles.
I come up with the same calcs as joed but do us both a favor and use #12 for you new line NOT #14. If the existing wire in the box is #12 you most likely have a 20 Amp breaker on the circuit. #14 is only rated for 15 Amp. SAD