Rule of thumb, double your cone area =3db
Double your power =3db
Upgrade alt, battery, wiring, big 3 = 1.1 idk for sure
But basically if you're doing a 43.9 with 1 12 and you add another 12, in theory you should gain 2-3db with as is. Anything you upgrade should yield what you're looking to get
Good luck with your goal