Any time you are using USN (or any other quick Reloading Normal Gun (AKA any Non-A variant gun)) for AA:
If total number of possible binds is greater than or equal to 15 but less than 20:
2 binds of HHE or HE (Or Firework shells if you desire) in the B Cabinet
Rest as AA
If total number of possible binds is less than 15:
All binds as AA.
For the few gunsets/ships that get 20 or more binds for AA:
60% of the binds as AA, 40% as HE for dual purpose/mixed role.
80% AA, 20% HE for AA heavy Dual purpose/mixed role.
Easiest of the remods for AA is the one with 6 guns, the Junsen?
However the other remods having the extra 2 guns can come in handy in some cases, however they make aiming interesting some times, especially when using Gradual Fire.