Phil - Please don't change subjects. It makes it impossible for
Google to archive a thread.

Here's a workaround. I've updated the demo file as well:

ftp://ftp.mcgimpsey.com/excel/ph_demo.xls

Private Sub Worksheet_Change(ByVal Target As Range)
Dim nRow As Long
Dim dV As Double
Dim dP As Double
Dim nStart As Long
With Target
If .Count 1 Then Exit Sub
If Not Intersect(.Cells, Range( _
"U19:U30,AD19:AD30,AM19:AM30,AV19:AV30," & _
"U51:U62,AD51:AD62,AM51:AM62,AV51:AV62," & _
"U83:U94,AD83:AD94,AM83:AM94,AV83:AV94")) _
Is Nothing Then
nStart = 10 - 32 * ((.Row = 51) + (.Row = 83))
nRow = (.Column - 21) / 9
dV = Range("V" & nStart).Offset(nRow, 0).Value
dP = Range("P" & nStart).Offset(nRow, 0).Value
If Not dP = dV Then
Application.EnableEvents = False
.Offset(0, 3).Value = (.Value - dV) / (dP - dV)
Application.EnableEvents = True
End If
End If
End With
End Sub

In article ,
"Phil Hageman" wrote:

Tom, Maybe you've hit on something here - it is obvious to
me, but wouldn't be to you, and I have been passing over
it every time I communicate with you folks. Makes me
wonder what else I'm not telling you...

It is possible some "P", "V", and "U" cells may hold no
values - this is a legitimate circumstance for the user.
Sounds like this may be my problem - the cause of blank
and #Div/0!answers. Can you: 1.) modify the code to work
around blank cells (dp - dv < 0), and 2.) make it work in
all three regions of the worksheet independent of cell
entries.

Is it possible for me to send you the worksheet via e-mail?