I think it's fine, as long as you realize that all you're doing is playing with numbers. It's not really going to yield you a much more accurate number. I'm saying this as someone with a spreadsheet tracking all kinds of variations. I love doing it, but I also know that none of it is real information.
First mini-goal: 5% - 209 - Met 04/29/11
Second mini-goal: 10% - 198 - Met 6/27/11
Third mini-goal: 15% - 187