Even better than an Excel macro, I think, is a web script. Nothing to download, nothing to install, no security risks.
So I implemented whirledtraveler's idea in a quick script.
Frequent Flier Goal Deviation Calculator
Comments welcome. Bug submissions are particularly helpful. (Email, via link on my page, may be more useful than numerous posts here.) I'm also happy to add more calculations -- just tell me what you'd like.