Limiting entry in text entry fields


I've got a calculator that uses triggers based on variables to calculate a grade level. I'd like to simplify data entry for my user. I'm thinking I'd like to use a drop-down list for the text entry, or limit the format the text entry field will accept.  Currently the user has to enter #-#, but I suspect I'll get different formats. 

Alternatively, I suppose I could write additional conditions using an OR operator, but I had trouble making that work.

Anybody have any thoughts on how to make it easier?

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