Custom Calculation script with 1 to 1 correlation

I am needing to put a custom calculation script that takes a value from a field.

The value from the field must be correlated against a set a numbers.

The Range is  0 to 40 for a student score.

The 0 to 40 range is assocated with a 0-100 scale.

The scale is not easily able to do a simple math computation to result in the desired value.

Student Score Range - Grade Value Range

0 -15,  0-59

16-19, 60-69

20-26, 70-79

27-33, 80-89

34-40, 90-100

I was thinking about doing a long if statement and do a 1 to 1 matching. Is there a way i can make this better?