After following Indira's instructions to select an aggregation, you want to select the aggregation, "Sum of Grades". This aggregation is set up so that each assignment is weighed according to the number of points you've assigned to it. For example, if you have four assignments and assignment 1 is worth 50 points, assignment 2 is worth 25 points, assignment 3 is worth 10 points, and assignment 4 is worth 15 points then assignment 1 is worth half the total grade (100), assignment 2 is worth one quarter or 25% of the grade and so forth. Then, if a student gets 88 points, s/he gets 88 percent.
Hope that helps.