Irregular hours worked - what is the annual leave entitlement?

Hi there,

My question is - if someone does not work a full 40 hours consistently each week, then what should I put on their file for MYOB to work out how much annual leave they are entitled to?? They think they should get 4 weeks at 40 hours per week each year.  I don't know if that is right, as they aren't working the right amount of hours in.  Thank you.