Gradebook

I am using Moodle 1.9.9 and I have a peculiar scenario for grading my courses.

I have a single category for each course using Weighted Mean of Grades.

I have 5 grade items under the Category. Grade Item 1 has a weight of 0, GI2 has weight .3, GI3 has .2, GI4 has .2, GI5 has .3.

The Course total is calculated as the weighted mean of the Grade Items GI2,GI3,GI4 and GI5. In addition, I would like the Course total to be deducted by some value depending on the value of GI1. For example, if GI1 has a value of 5, course total should go to the next lower grade letter.

Has anyone come across such a scenario? If so, please help.

Does grade calculation have a conditional function like Excel if()? One can refer to grade items in calculations but is there a way to refer to the Course total?