> I'd get D >= X >= C+, where X is my grade

Mostly as a curiosity, what you've written here is impossible, lol. I think you mean your grade would range from a D to a C+, inclusive. But you have written that your grade would be greater-or-equal to a C+, yet less-than-or-equal to a D, which would be.... impressive :D

edit: Closed intervals are great for communicating this kind of thing in general. You could say your grade is in the interval [D, C+], where the brackets indicate that D and C+ are included in the options. Compare to the open interval, which would indicate that D and C+ are excluded -- ie (D,C+) includes only the entries D+, C-, C.