Numeric scores that are sent/imported to a letter grade gradebook need to be converted consistently and accurately.
Example, If external grades are 93.3 (A) and 83.3 (B). The average is 88.3 (B+). Letter grade gradebook should look up and convert 93.3 to a letter grade A (95) and 83.3 to letter grade B (85) and average the grade as 90 (A-).
external tools and import can provide point scores higher than points possible, for example. In this case the provided value will be assumed to be a top lettergrade possible and saved. For further calculations the lettergrade value for that tops score will be used.