From ratings to grades

I have decided to switch from giving the movies I see star ratings to giving them letter grades. On some level I hate doing this, but it seems less absurd than drawing distinctions based on quarter stars, or even, in some cases, a third of a star. Roughly speaking, the following equivalencies apply:

A = **** (Outstanding)
A- = *** 1/2 (Excellent)
B+ = *** (Good)
B = ** 1/2 (Decent)
B- = ** 1/4 (Some merit, but seriously flawed)
C/C+ = ** (Mediocre)
D = * (Awful)
F = No stars (Unwatchable)

I realize the above isn't exactly to scale, but it reflects the fact that most movies I see are going to fall somewhere in the B range, that is, between 2 and 3 stars. That's just the way it usually works out.