Budget Woes

The title to this post may be a bit misleading, because it's not woes as in we can't possible plan a wedding on what we had planned to spend. Which is true. But this post is about the fact that I feel completely stumped trying to figure out how much things cost. I mean I have my Real Simple Weddings budget sheet that tells me what percentage I should spend in each category. So, I know my target. But what I don't know is how much will alcohol for 150 people cost if we bring our own and can get it at a discount. It makes it hard to figure out what budget we are working with for catering when I am at a loss to plug in a number for alcohol. Or centerpieces, if we DIY. Am I just being too detailed? Should we just build up individual elements as we get estimates and just work toward our targets. Has anyone else had this problem?