To calculate your grade at any time during the term, I divide the number of points you have earned by the number of points you could have earned. That number is your numerical average, in decimal form. Use it to find your letter grade (or just wait for your PR, and I'll email it to you).

If your numerical average is in this interval...
...then you have a(n)...
[0.9 , )
A
[0.89 , 0.9)
A-
[0.87 , 0.89)
B+
[0.8 , 0.87)
B
[0.79 , 0.8)
B-
[0.77 , 0.79)
C+
[0.7 , 0.77)
C
[0.6 , 0.7)
D
[0 , 0.6)
F