I can determine if the change was in the correct range. I do not know
how to verify the contents are numeric, and then adjust the value in
the adjacent cell.

Private Sub Worksheet_Change(ByVal Target As Range)
If Not Intersect(Range(Target.Address), Range("d6:d19")) Is Nothing
Then
MsgBox ("Change in D6:D19 range")
ElseIf Not Intersect(Range(Target.Address), Range("f6:f19")) Is
Nothing Then
MsgBox ("Change in F6:F19 range")
End If
End Sub