This is pretty much what EAM and I have been saying. 10/12 is 83%. Ad hoc roster is another phrase for flexible and random and per EAM the block roster may come later. There may be times of long blocks and short breaks but the total days free of duty are contractual and they do achieve it.
To try to nail this down. A full timer has 162 days pa free of duty which is 42 days leave and 120 days off. This is contractual, so average 10 pm plus leave. 10/12 you lose 7 days leave so 35 leave. Gain 60 part time. Days off is 10pm for 10 months is 100. End up with 35 leave and 160 off spread through the year. This my understanding and I’ve been playing part time games since 2008.
Hope this helps