You could use an event macro.
Assume the range of interest is A1:A100 (adjust to suit) and cell to change
is C10.
Amend logic to determine add /subtraction
Select the sheet where you want this to happen.
Right click the sheet tab and select View Code.
Paste this code into the window that opens.
Private Sub Worksheet_Change(ByVal Target As Excel.Range)
On Error GoTo ErrHandler
Application.EnableEvents = False
If Not Intersect(Target, Me.Range("A1:A100")) Is Nothing Then
Range("C10")= Range("C10")+Target.value
End If
ErrHandler:
Application.EnableEvents = True
End Sub
Hit ALT Q to return to Excel.
HTH