I realy don't wont to reinvent hot water, but I took it as a task to remember all those math lessons from school. So I came up with this Microsoft Excell 2003 XLS or OpenOffice Spreadsheet.
FTP user:diyrc pass:diyrc
It basicaly calculates an airgap and maglen spacer thickness based on your stator and bell diameter, and magnet dimensions, number and thickness.
You can play with it to get optimal values for your motor.