I need to have an annotation that is driven by a complex long equation.

Basically, I need to create a letter (A - ZZ) that is driven by a numeric result. If the value is

num >= 6.25 and num < 6.75 then the letter is "A"

num >= 6.75 and num <7.25 then letter is "B"

Does anybody have suggestions to:

1) Make that formula to do all the greater than/less than's simple?

2) Create an annotation that displays a letter based on a numeric value?

This is converting a distance between two features into a simple letter for setting a machine in production.