I'm on the univeristy computer now and having problem to disable First Class too.
i made this simple way.. i dont know if you could implement.
First class: (formula / ACFTtotalPAX) <--- (optional)
C class (formula / ACFTtotalPAX) <--- (optional)
Y class (available seats with the resulting space)
Then if they entered a input/data automatically set the option, if the value is 0 or NaN means option deactivated.