I wanted to do an online "point calculator", then store the daily points in a database...
In addition, take the books they already have and put them online, too. With all the points for Taco Bell, McDonalds, etc.
I would recommend you do it "server-based", and don't let them see the server components.
Set up a demo where they can log in and go the functions from your spreadsheet.
They will ask for the "how do you do this" and possibly sample code, your answer should be that they need you to do it for them. Make some requested changes (3-5 changes or enhancement) to "prove you can do it", but keep them server-based, so they can't steal the answers.
Good luck.
Glen Austin