When a cell is changed, could you use the

Private Sub Worksheet_SelectionChange(ByVal Target As Range)

event to validate the range? (Do you know what I am talking about and
how to do that?) After you build your worksheet, enter the macro
system (alt-F11), double click on the name of the worksheet in the
project explorer, select 'Worksheet' in the first drop-down box, and
program away.