Trigger for Calculated Field

I need to figure out how to display a warning if a certain event
triggers. The screen is Financial Management, Accounts Payable,
General Operations, Invoice Entry. The tab is the summary tab. The
field I wish to check is the field "Variance" which seems to be a
calculated field. I wish to display a message if the variance is
either over $5 or under -$5. I see how to add a warning to a field
that you actually have to fill in, but I am unsure how to add this
warning on a field that is calculated.