@jsweeny
There is no way to create calculated grades within the native interface. There are some ways to work around it, but they all require extra effort.
Ultimately, you would need an extra assignment to hold the grade that you want. Then you tell Canvas to not use the other assignments for grading. You can do that by checking the box to not count this grade towards the final. If you use a weighted gradebook, then you can use an assignment group to hold the initial scores and make that assignment group worth 0% of the grade and it doesn't matter if you check that box. If you use graded Classic Quizzes, there is no option to make it not count, so an assignment group worth 0% is the route to take.
I used the 0% assignment group in a course this spring. I had all of the assignments from an outside homework tool go into that group. I gave the students full points for taking a pre-test, regardless of their score. If they scored above 80% on the pretest, they didn't have to do any of the homework and I gave them that score for the exam. If they did do the homework, then I used that score instead. If they took the test and that grade was higher than their homework grade, then I used the exam score for their homework grade. There were some other techniques in there, but all of the original scores from the outside system were kept inside Canvas.
Within the normal assignment groups, I held a single score for all of the homework for the chapter, an assignment for the exam score, an assignment for taking the pretest, etc. The stuff that was used to calculate scores but wasn't used directly went into a "non-graded" assignment group worth 0% of the grade.
Since I can do a little programming, I wrote a script that would download all of their grades from Canvas, compute the scores and update Canvas with those so I didn't have to mess with it. An alternative is to use the Gradebook export feature, bring the grades into Excel, create the value of the calculated field, and then import the grades back into Canvas.