Agree with BtS. As far as the CAA are concerned, leave days would count towards your 12 week totals. As would days off away from base. Some carriers have a pro-rata system for leave towards the days off in 12 weeks, but this will be through the scheduling agreement. Typical example is 3 days leave counts as 1 day off towards the 24 total.
Watch out next year, btw. Most of the fixed period rules in CAP371 are planned to change to rolling periods so it will be 7 days off in any 28 day period.