As you add up your points on the right side of your grade worksheet this equals your current grade. If you add up all your points and that equals 70 points, that is your current grade. A 70 = C.
You do not add up all the points and average those out of 90. Again, there is no "average" in this grading system.
If you were to have 70 points, you should take the final to try to get 10 more points to put you at an 80 which would equal a B.
So again, you add up all your points on the right side of your grade worksheet and this gives you your current point total which is your current grade. The final exam (if you take it) will add to that point total which will improve your grade.
But again, there is no AVERAGE in this system. You do not average your current grade out of 90. You add up all the points you have and use the system below to determine your grade:
A 90 - 100 Points
B 80 - 89 Points
C 70 - 79 Points
D 60 - 69 Points
F 59 Points or less