Lot Grade

We are trying to make use of Lot Grades.  I'm trying to understand how I can assign a couple of different acceptable grades to a customer when they are in in sequence. 

For example, we may have grades A, B, C, D, & E. 

Grade A is acceptable by all customers and would be the preferred grade.  However certain customers may accept grade B and D for example (note the grades indicate differences in test results not necessarily quality of product).

If I try to add a 08 preference it seems like I can only put in a range but the customer would not want C grades.  It also seems like I cannot add two records as I get duplicate errors.