Grading???

While grading I want to determine if my students have "mastered" the content. The problem is what is "mastery"? I define "mastery" as a students ability to demonstrate through verbal and algebraic means an in depth understanding of the problem. Mastery could occur in any classroom activity, however all assignments are not equal. Currently I use a 60-40 weighting, 60% assessments and 40% classwork. This has been a method I have used my entire teaching career without question. I have begun to wonder is this really the best way to measure "mastery"? Next year my school will begin to move towards project based learning where "mastery" can be seen in a final product. I love this idea, but I do not believe my current 60-40 system would work. Perhaps I will move to a total points system that will vary the weights each marking period, but will reflect the type of work being done in class. I believe the method and ways that you grade should based on what best reflects the work students are doing in class.