Essentially you can talk any room block down, just make sure it is in your contract. I guessed we had 100 guests, I only had the 10 night guarantee put in because I wasn't really sure how many people were actually coming, plus lots of Floridians. You can always add rooms and just up your guarantee. Closer to the date is more difficult because of available rooms. However if you guests book early you can make sure you have enough.
Also if you get the 25 nights the bride & groom get 1 free night at the resort of their choice.