Trouble with Worksheet Change when Target Cell Changed by Formula

[phelbin]

I have a list of checkboxes that have referring cells (TRUE/FALSE) in columns X and Y. In cell Z3, I'm counting the number of TRUE values in columns X and Y. If it goes above 9, then:

        If Range("Z3").Value = False Then
            MsgBox "ERROR: You may only select 9 Contact Tags"
        End If

My problem is that cell Z3 change is changed by referring cells, which are changed by the checkboxes. I've tried a half dozen different ways to get the Worksheet_Change or Worksheet_Calculate events to trigger, but no luck.

Any ideas?