Honestly, I just think two scales for Magic Ability is redundant. Not only because it is a subjective enough thing that it always requires a detailed explanation anyways (since not everyone has magic and those that do vary significantly), but because having two scales invites confusion and stupid arguments like this.
An example of the way I would see it: Rin would have A magic ability, because within the magic system she uses (modern Magecraft) she is good enough overall to qualify as a top user in every way that counts. But Medea would also have A or A+ because within her magic system (High Speed Divine Words and Age Of The Gods Magecraft), she is also that powerful.
Yet Medea is a more powerful magic user than Rin because they way they use magecraft is inherently different. There are things Medea can do that Rin simply cannot. Just as there are things... say, Uchiten can do, that Dunkelzahn can not and vice-versa (because of one being an original character with a loose and powerful magic system, and the other from an existing setting that also has very powerful magic with unique uses, but also very strict limits).
Now, this kind of difference is something that has to be specified in the explanation of the character's magic ability following the rank. And that's a thing that already benefits from detail, so it's not really as hard as that.