THE SCHOOL OF HARD KNOCKS counts a fair number of independent meeting planners among its graduates. Lessons on finding and retaining clients, budgeting time, and running — and growing — a lucrative business don't always come easy. One of the biggest challenges is the inexact science of figuring out how to charge for services. Should it be an hourly rate or commission? A set fee plus commission? A project fee determined by the size and scope of the project?