Leslie,
First off, my "opinion" is there should not even be a category for extra credit. Good students won't need it and poor students will abuse, at the expense of the regular assignments.
One way to approach assigning weights is to look at every category of graded assignment and estimate is overall worth (contribution) towards student learning. Usually only one measure (i.e., one giant final exam) would not be reasonable. But if you had categories for say, tests, quizzes, homework, project, discussion, and skill oerformance (if appicable), then you look at your learning objectives and ask the question: "to what… >>>
