This problem does sound best handedl with field validation on the salesforce side. So you are on the right track. But you might also explore some client side validation that Zach suggested a few weeks ago. The scenario is a little different, but I think you could use concepts to work toward your solution. Here is the link: https://community.skuid.com/t/magic-real-time-error-warning-rendering-using-javascript…
(You will have to ignore my snide product development comment)
