macro not working if formula bar is used

I have a piece of code as follows

If Range("FY05_deducuction_percentage").Value < 0.95 Then
    MsgBox "FY06 percentage has dropped below 95% of FY05 budget" & vbCrLf & "Please adjust the last figure entered!!!", _
    vbExclamation, "Please edit the last number"
    Application.Undo

it works fine,, until somebody enters values in the formula bar, then the macro won't notice the value is less than 95%.. How can this be corrected? Thanks!

Re: macro not working if formula bar is used

Thanks for the reply.. Hope this is helpful

Private Sub worksheet_change(ByVal Target As Excel.Range)

I'm assuming there is a different/better way to activate it? Thanks!

Re: macro not working if formula bar is used

just an update.. I tried using the following:

Private Sub worksheet_change(ByVal Target As Excel.Range)
If Range("FY05_deducuction_percentage").Value < 0.95 Then
MsgBox "FY06 percentage has dropped below 95% of FY05 budget" & vbCrLf & "Please adjust the last figure entered!!!", _
vbExclamation, "Please edit the last number"
Application.Undo

but now I can change a figure, it will go below 95%, and won't give a message until I move the selection. which sounds fine,, but now if I just change something and then hit close, save. It allows the change, then next time the sheet is opened, the message comes up, but it can no longer undo the change.. Any ideas?? Thanks!