Have you tried the AA Naval academy mission?
Those swordfish planes should go down after just one or to hits when you´re on vanilla 1.4.
Like i sayed i never succeed taking down those fighters in that mission.
But if the swordfishs are easy to kill in your game everything should to be alright with your 3.7.
I´m no good flak gunner at all cause i don´t fight planes.
Instead i try to diving away, so i have not much tips for you.
But i think the 3.7 is best used on big slow targets like bombers (fishingboats

), the 20mm are better at used at fighters.
Please excuse my english!

Edit:
try not to use the magnifigation when you´re manning the guns for yourself. I found it´s harder to aim as without.