Calculate late penalties for assignments. Merged

I assign late penalties each day an assignment is late (10% per day). It is very difficult and time consuming to go through 120 assignments and manually calculate how many days late the assignment is and how much to deduct. If this could be automated to automatically show the grade with the penalty calculated (much like MathXL does) it would save me a ton of time. This would, of couse, require a check box and option for the teacher to input the late penalty percent in the setup of the assignment.