You cannot use Real/Percentage/Letter, since this is a manual column set to "Text" type.
As Ron mentioned, you can go to My Preferences > Grader report > General > Show quick feedback - YES.
That way you will be able to type your comments quickly in the Grader report.
For students your comments will appear in the Feedback area for this grade item.
You can also choose to click 'Edit grade" icon near an individual student grade, to go to advanced con. You have included that in your own screenshot.
That way you will see even more controls/feedback area for the grade of some student.