Add option to upload custom grading scale

My school has a weird grading scale and the webapp keeps showing that the grades aren't the same as calculated.
Ex/. 
An **A** at my school is **3.4 - 4** or **90 - 100** while a **B** is **2.8 - 2.39** or **70 - 89** with **NO +'s or -'s** at our school.