If you plan to continue teaching, you will have to develop a better system than "intuition."
That just won't cut in today's push for quantifiable, accountable grading at every level. I
never weighted my grades into percentages; it was strictly point-based and pretty easy:
practice assignments (sketches, trying out new techniques) earned 50 points, and "big"
assignments earned 100. Sketchbook work earned 25 points.
Here it helps to have clear objectives--what exactly are the students to learn? If they are
to produce a painting using a palette of split complementaries, then you will assess how well
they achieved that and what would constitute an A, B, C (on a point scale)--and let them know
ahead of time what an A looks like. When faced with a pile of student work, I would sort it
quickly and roughly into A, B, and C piles, then go back and determine exactly where on the
point scale they stood. Sometimes a piece would be recategorized to a different level.
We all have different ways of grading and you will have to pick the things that suit your
style. If you're really stuck I could send you some samples of rubrics.