I have the follwing code which sets a formaula in a specific column

With Margin
.FormulaR1C1 = "=SUM(RC[-1]-RC[-3])"
'.Value = .Value
End With

However some of the values in the C[-3] column are blank and so neagative
figures show as a result. I need to amend the formula to include an if
function which says that if the [-3] column is blank then return a zero as
the result