|This Data Validation formula, entered the Custom field seems to work. I don't know if it's the most efficient method, but it seems to meet your criteria.|
The assumption is that your Categories are entered in A2 and the Percentage is entered in B2.
Note: Once a value in B2 is accepted as valid, changing the Category in A2 is not going to automatically make the Percentage in B2 "invalid". The Data Validation is only performed when B2 is edited.
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message edited by DerbyDad03