Dave, just remember you can't make everyone happy, no matter how hard you try. If you made it 100% fair, you'd have 30 different classes, and everyone would be in their own class. I would suggest you make it as fair as YOU think it can be, and stick with your decision no matter who, or how many people complain. It wouldn't hurt to post how the classes will be split before the event, that way people can plan to make modifications or not, depending on what class they want to run.
P.S. I know what a pain in the A** an event can be, and you guys are doing an AWSOME job.
