The up arrow on my keyboard died. One place where I often use it is when entering course grades.

Before if I had a bunch of graded tests for a class and need to enter them into my grading program I would simply click the blank for the student, enter the grade, then move up or down with the arrows to the student for next test in my pile.

Now I can move down with the down arrow but to go up the list I have to reach over to my mouse to click the correct spot which takes a bit longer.

My question is: for a class of size n, how many times should I expect to have to use my mouse to move up the list? (Assume the tests are in a completely random order.)