The key here is Annual Leave and the fixed roster pattern for the Leave.
75% of 28 is 21. That’s 4 blocks of 5 and a random day.
On a fixed 5 / 4 roster 5 days of AL is 13 days off.
4 blocks of 13 is 52 days plus the odd day is 53.
So by my reckoning 9 months work is reduced to about 7 months work after AL.
100 hrs per month so 700 hrs plus a bit is the max you can do.
Thats my back of the fag packet calc.