Here's my mapping of scores to letter grades. I don't do B+/A's here -- what I do is convert each exam grade to the same scale as the labs, and then average them all.
This is a Tukey Plot, which has lines to the max and min, yellow box denoting
the 1st and 3rd quartiles, hash marks at the median, and dot at the mean.
In the "answers and grading", there are histograms of scores for each question.