But you can book the first nights of your stay at 8am when online booking opens at 7/11 months with the points you have in the account already. Then you can call MS to book the last night(s) using one time points. This would lower a bit the risk of the room you are interested in being booked between 8am and 9am.
If you book at 11 months you should be sure to get the last night (unless there is some software problem), while at 7 month you cannot be 100% sure.