I would probably develop a formula using values from a table that would indicate upper threshold, lower threshold, percentage, etc.
Develop your formula using variables. You can hard code the values of your variables while you develop your code. Later, you can place the values in a table, where your variables can reference them. Create the table after testing that your function works.
If you use a case select to call different functions, those functions will probably have another Case Select statement and or an If Then Else. The tutorials I linked to happen to go over Functions, If Then Else Statements, and Select Case Statements. It will take a while to go through the Introduction to VBA series of tutorials. But, I believe they are relevant to what you are doing.