Need dashboard prompt based on field with complex formula

Summary: I have a report on a dashboard that has a field that gives me the highest level of education obtained by first ranking the degrees on the employee's records and then returning the top one. It then groups these degrees into categories, such that all types of bachelors degress return just Bachelors and all associates or technical degrees returns just Associates/Technical. I need a dashboard prompt for this field but I cannot get it to work with the formula I have used for the report.

I tried to use the field called Highest Education Level, but this is null for all employees.