I'm having a bit of a break (hoping that boredom will move me to make a start on some projects that I've been avoiding for ages). I'm just a hacker nowadays in Excel, gnawing away at problems that interest me. I see Shaun replied on another thread; I think his existing sheet does most of what you require (linked in his sig).

If there's anything additional that you're both struggling with, post to this thread and I'll have a look (if Shaun can't do it, it'd surely be an intriguing challenge!)