That's OK, but the site would be a lot better if you could put in the ratio and it would give you all the combinations of pulleys that can be used. That page is really just for calculation the belt length for a give pulley combinations. When you're trying to get a gear ration based on pie for a belt driven system it's takes a little bit more work.
I found this bit of info-
http://www.sdp-si.com/D260/PDF/part1.pdf