Bex Formula Question - Character Data

We are developing a query to display rankings of material. If the material sold is in the top 80 % we would like to assign the value of A, the next 15 a value of B and finally C for the rest. We thought we could use a formula, but it appears that you cannot assign character type data in the formula i.e "A". Is this the correct approach or is there a technique for doing this type functionality?

Thanks for help.