As @shoover hints at in their comment, the question assumes that you've already answered the question of why you are assigning letter grades at all. Assuming the answer to that is something along the lines of "because I have no choice" or "that's just how it's done", here's how I approach it. (Perhaps useful info: my college has a brief description of what letter grades should mean across disciplines. For example, a B is an indicator that the student is ready---and encouraged---to pursue further studies in the discipline.)
I don't use numbers at all any more, and grade assignments like (I assume) my colleagues in the humanities do. Does the work demonstrate mastery of the material? Put an A on it. Is it littered with errors, both small and large, but with evidence of a reasonable amount of understanding of the basics? C. Of course, there's written feedback on individual errors/questions and the assignment/exam as a whole as well. This written feedback should, and usually does, correlate with the grade.
Course grades are then compiled from these individual grades in some somewhat arbitrary but pre-defined and known way. (Best 6 assignments from 8 count 60% and the final exam counts 40%, or whatever.) I also reserve the right to move a student's grade up or down by upto one letter based on promptness of submission and class participation and whatever other sundries pop up during the semester. (This is much more often a boon for doing something not explicitly assigned credit rather than a dock.)
In my experience, this bypasses a lot of the concerns that seem to have driven the question.
[Bonus anecdote; related but not strictly part of an answer: I studied and then taught a little in the UK in places where 70ish earned an A and a passing mark was around 40. I moved to the US and started teaching without knowing about the usual US system. In my first Calc 1 assignment my students did well with some tough (in context) questions and most scored between 60 and 80 (so, I thought, at least a B- and some really strong As). They were not happy. This is part of what prompted the switch to my current system, but I like it regardless of its origin.]