I'm still confused by the room block and our wedding is this month!
The way I understand it is that you can block 20% more than the rooms that you are required to fill. For example, if you want to guarantee 40 nights, you can block 48. It's not much of a margin and we also felt like we had to find out exactly who would be staying where on which nights. At the end, our guests were completely unpredictable. We thought most of our guests would stay in the value and only one couple is staying there. We couldn't even predict who would come early and who would stay late.
However, despite these issues, we didn't have a problem adding nights when we went over our room block. Everyone was able to get the discount. As long as it's not a very busy time of year, and people book before the 30-day mark, you should be okay.
The best room block advice you can get is to book one value, one moderate, and one deluxe.