I think it's a good idea to implement Simba's spreadsheet as a web-based app, but you might want to link to the original spreadsheet and make your tiny "credit" section a bit more obvious.
Personally, I find the spreadsheet easier to use. I couldn't easily modify my scenarios with your app. Try putting the inputs on your output page, and make it easy to resubmit with some changed input.
Favorite ERF quote: "I'm not going to waste my time on someone who's more interested in being stubborn or obtuse or intolerant." -- Nords
Favorite ERF error message: "Sorry Nords is a moderator/admin and you are not allowed to ignore him or her."