I agree with others here, and suggest you get Cooper's book 'About Face'. Regarding your particular application, I daresay it's important to make it easy to use the application with the business rules in place.

Where it may have previously been possible for users to type in any value they like, you have tested these values for correctness AFTER the work is done. Not knowing anything about your app, I might ask how would you design the interface differently so that correct entries are easy to enter, and incorrect entries impossible to enter? This might require more picklists, disabled entries, or perhaps even an entirely different schema for the user to relate to the data.

A big design job - much bigger than just plugging in some error-checking code into an existing application!

FWIW....