Disney allows you to block 20% more room nights than you "guarantee". So if you only want to guarantee the 25-room-night minimum, you can have 30 room nights in your room block. If you and your guests book less than 25 room nights, you are required to pay for the extras, to meet the minimum. If you and your guests book at least 25 but less than 30, that's fine.
Now, if you want to reserve more than 30 room nights in your block, then your minimum goes up, too. I started with a 60 room night block. That meant I was required to book 50 room nights. One thing you can do is increase your block as it starts to fill. I recently called group reservations and found that I already had about 90 room nights booked. Since it's still more than 3 months until the wedding, and the vast majority of our guests (who say they're coming) haven't booked rooms yet, I increased my block to 120 room nights. (So we have to have a minimum of 100 room nights booked.) That was a pretty conservative increase, since it meant that only a couple more guests need to book rooms to reach the minimum. I may end up raising the block a bit more, too, if I can.
Just to give one more example: if, in a few weeks, I find that our guests have booked 150 room nights, then I know I can add 20% of that (30 room nights) to our block without any risk.
Hope that's all clear. Oh, one more thing. (Which may complicate my explanation unnecessarily.) If guests book rooms independently (i.e., not through disney weddings group reservations), those room nights count toward your minimum but don't use up any of the room nights that were blocked. In my case, some guests booked rooms at the rack rate instead of the discounted rate, to take advantage of the free
disney dining plan offer this fall. I get to count those room nights towards my minimum without subtracting from the blocked rooms.
All that being said, I am worried about guests not getting rooms. I wish people who know they're coming to the wedding would hurry up and just book their rooms! It's not like the rate's going to change... I only just got my brothers to make their reservations last week. My mom would have killed them if they had waited too long and had to stay at some other resort
