You would have to create a "Weighted Column". The only issue with this, is that you would have to go in and create a percentage for each item/category and make sure each item in the grade book is assigned a category.
In order to do this,
Step 1: Click on "Create Calculated Column" 
Step 2: Click on "Weighted Total" Column
Step 3: Name it Weighted Total or Overall
Step 4: Change the "Primary Display" to "Percentage"
Step 5: Change the "Secondary Display" to "Letter (No Rounding)"
Step 6: Scroll down to the section "Categories to Select" and move selected columns to be included in the grade book.
Step 7: Enter the percentage amount for each item moved over.
Step 8: In the "Options" section, make sure it is visible to students.
Step 9: Click submit
Now we must make sure that it is set as the "External Grade"
Step 1: Click the arrow next to the name of the column you just created
Step 2: Click "Set as external grade"
When you add a new column, it automatically adds to the end of the grade book. Please see "How do I show/hide columns in the grade center? How do I rearrange my grade book items?" by clicking the link provided after you have continued reading. http://gtclibrary.libanswers.com/FacultyFAQs/faq/115947?m=p