Modify teh present formula

[john55]

Hello,

I'd like to have a formula in order to subtract hours take into account 1 condition.
-If he/she is on duty (the simbol can be: N, N1, N2, N3, N4) in the last day of month then subtract 6.
I use for calculation this one:
Col AQ: COUNTIF($C12:$AG12,AQ$11)*AQ$10
for example, the first guy works 4 nights, I multiply by 8, result 32, now because he works in the last day of month I need to subtract 6 hours, so the result should be 32-6=26.

Attached a test file for your reference.
Thank you very much for your help!