I had a solution for automating this in a similar situation, where the exams were outside of Canvas.
Setup: Four Canvas assignments
1) Assignments, worth 40%
2) Midterm, worth 30%
3) Final, worth 30%
4) Re-weight, worth 40%
I uploaded each exam's grades twice, once on their respective specific assignment (midterm or final), and another in the re-weight assignment.
In that re-weight assignment, I set it to drop the lowest grades,
That way, the lowest of the two will be worth 30%, the highest of the two will be worth 70%.
And that has the advantage that students see the calculation working inside Canvas.
Still trying to figure out a way to do this with exams being Canvas quizzes, though, since I don't have a way to "repeat" the grade.
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