I use it frequently. When you click on the link for Stu's Calculator it will take a minute to load and come up as a live link. I don't think you can download this program so you can work on it without being connected to the website and on the internet.
Some of the boxes have dropdown arrows like Arrow shaft size, bow type, feather, strike plate thickness, string material type. Click on the arrow and choose the closest to your set up. For the other boxes, highlight the information in them and delete it and type in your specs in decimals where called for.
Use the Rev 12/25/10 revised 2/14/11 (v2). It's pretty easy to use and I'd advise reading the instructions first.