Two ways that might work:
1) Don't try to set up the scale in the quiz. Instead do it in the gradebook. Add new grade item, set it to equal the quiz grade, and display using a scale.
2) Export all the data from either the quiz (Quiz admin -> Results -> Grades) or from the gradebook. Do whatever processing you like in Excel or another tool. Then, optionally, re-import the results of that into a custom column the gradebook.
The second option is more work, but is quite flexible if all else fails.