Negative Grades In Moodle

We had an interesting question from an instructor:

What I want to be able to accomplish is to be able to input negative values to a grade.

In my class attendance is required. If students have perfect attendance they receive 10 points extra credit toward their final grade; if students miss 1 or 2 classes, their final grade does not change; and any subsequent absence, they are penalized by 10 points to their final grade. 

What I want to do on Moodle is to add an assignment and call it attendance where the value of the assignment is 0. I would add 10 points if students earned perfect attendance, and I want to subtract points from the final grade when they've missed more than 2 classes. However, Moodle will not allow me to input a negative value. How do I subtract points from the final grade without taking it out of previous assignments? Is there a way for you to allow me to input a negative value?

Eric Bryant delivered a terrific solution:

As this is a somewhat unique way of grading, my suggestion would be to create a manual gradebook item that has a maximum value of '0' and a minimum value of '-10'.  

You will also need to create an additional manual gradebook item with a minimum value of '0' and a maximum value of '10' and mark this one as Extra Credit.

Setting it up this way will allow you to assign every student on the first gradebook item a zero (100%), or something less up to -10 points (0%).  The second gradebook item will be for those students earning the extra credit.  As the item is marked as extra credit, only those earning 10 points will be awarded and it will not impact any other student grades.

Remember these two manual gradebook items should not be within a category if you use categories.