Reserve is 18/12 (13 off in a 31 day month). The basic structure is 6/4, but you can waive the 4 in a row to get different blocks. Minimum days on reserve in a row is 4.

The DTZ doesn’t affect your usage other than it typically brings the reserves available down to the required buffer. Usage obviously depends on base, but expect to get used at least 4 days each block.

Yes, you can X/Y for premium, but you can’t do a straight pick up from OT.