Here is what I did. For me there is much more to the gearing than just the total spread of the gearing. My desire was to have main gears centered around the middle chain ring. I then wanted a range of granny gears at least 6 for riding the rolling hills around here without always shifting in and out of the granny. To get that I had to go with a wide spaced cassette and that lead to wishing I had close spacing when road riding. That determined my big ring size to fill in the gaps.

No one else I know has a setup like this but that was my logic and I like the results. Link below.

http://www.bikeforums.net/showthread...4-28-32-36)-o)