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I do not assign grades on an absolute scale because even after many years as a professor it is not easy to
calibrate the difficulty of exams "just right".  The typical average letter grade in my classes is about a B/B+,
but in some cases it is higher when I feel that student's performance exceeded my expectations.

Here is an example of grades from the last time I taught a large undergraduate class (Physics 24, W2018).

The "combined" score (scale 0-100) was calculated based on the final (50%), the midterm (35%), and the
homework (15%). The four distributions are shown below.



Then, based on the combined score, I assigned letters grades "roughly" as follows:

• > 96 A+
• > 84 A
• > 80 A-
• > 76 B+
• > 70 B
• > 66 B-
• > 63 C+
• > 59 C
• > 56 C-
• > 54 D+
• > 50 D

I say roughly because I bumped up letter grades of several students as follows:

• if the combined score was very close to a boundary, e.g, 79.9
  
• if the performance on the final was significantly better than on the midterm
• if the combined score was hurt by a "poor" homework score

These were all judgement calls that go in favor of the students.

In this case the mean combined score was 78/100, which corresponds to somewhere between B+ and A-.
(That class actually did "better than expected").