Well, you need to start with finding out how many working days or duty periods you need to man
pr. month or whatever period you are going to use as your reference.
That should include all flying, training, reserved/standby duties etc.
Once you have you have to find out how many days in each period you can use each crewmember, taking into accound days off, leave etc.
Quite a bit of work, but makes for an interesting excercise.