Grading Scale

I need to do some research on grading scales. As it stands now, my grading scale is this - themes/essay (60%), Quizzes/SSR (5%), writing process (20%), journals (10%), and daily work (5%). At first I thought this scale was fine. And it probably works about 90% of the time. It puts the emphasis on the final products (their essays) and motivates them to use their class time wisely to write and work (hence 20% for writing process). We do a ton of grammar and other worksheets, but I'm not convinced that makes anyone a better writer, so I de-emphasized those. But the problem that arises, and it's usually one or two kids per class, is that some kids will do relatively well on their first three themes (description and narrative) and then really bomb on the more complex themes (analysis and review). Yet, because they have worked hard and turned in everything and kept up with their journals, they squeak by with As when they writing is really C quality. This drives me nuts. I'm not preparing these kids for writing out there in the real world or college. Maybe I should switch to an overall point system. But I know nothing about that. Suggestions?