In the Assessments & Grading module > Grade Setup > Weighted Subject & Category Settings, you can identify the grading scale that you want to use for the Overall Score.

This grading scale will be used for the Overall Score display of all your subjects.

Now, if you want to use a different grading scale for certain subjects, just click on the setting - "Manage Weights and Overall Score Grading Scales"

Once you click on that link, you will be taken to a page where you can set a custom grading scale for the overall score of each subject. Here you can set the grading scale for each subject as you like.

Note that if you didn't set a custom setting for the overall score, then it will just follow the default settings that were set from the main Grade Setup page.