I've honestly never understood the letter system for scoring. Is not intuitive and is open to so many interpretations.
Once I had to use letters, and at least for the case of failing the student it was quite straight forward.
Tests, homework, and every piece of work was graded in a scale from 1 to 10, at the end I just did a weighted average for the different elements.
So I ended up with a number between 1 and 10, I consider someone who has passed the class as someone who has at least more of half of the knowledge imparted in the class, so that would be having more than a round 5.
So the threshold was set at 5.1, I would fail those that got less than 5.1 and just prorate the rest of the letter upwards.