Ok to ask about grading scale?
Yes, it is ok to ask and to say that you really need to know how you are doing. But, I'll guess that  you won't get a specific answer, or, at least, not as specific as you like.
It may be that the &quot;scale&quot; is really just &quot;preponderance of evidence&quot; for the various grades. After all, the grading system in the US is fairly crude with only about five levels of grades. So, since you don't have a percentage score recorded at the end or a total this might be adequate for purposes of grading.
I would hope, however, that the instructor gives you some written feedback on what you write as well, so that you can see how you might improve.
But, yes, you can ask, and it isn't rude.