Alert user when rollup exceeds threshold

I am on Sugar Professional v9 on Site.

I have a custom module related to Opportunities, call it Evo.

Evo has a percentage field (an integer with max value 100).

I also want the sum of all the Evos related to an Opportunity not to exceed 100.

So I would like to, at the very least, Warn the user if the sum exceeds 100.

I tried creating a rollup field in Opportunities and have an on-change event on that field, to check the total and show a warning alert if it's over 100. But the on-change will not trigger when the formula field is updated.

Any other thoughts on how to do this?

thank you,