Convert Sumproduct Formula To Macro

The attached has a sumproduct formula that is slow down the calculation of my workbook. (I know there is also an array formula-that's another thread). I'd like to use a macro to fire on the worksheet change so I don't have the heavy recalc burden. The formula is in worksheet "Database" in column H. So far:

Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Cells.Count > 1 Or IsEmpty(Target) Then Exit Sub
If Not Intersect(Target, Range(Range("rReason").Offset(1, 0), Range("rReason"). _
Offset(UsedRange.Rows.Count + 1, 0))) Is Nothing Then
Target.Offset(0, 1) = Application.VLookup(Target, ValList.Range("ReasonLkUp"), 2, False)
'This formula below needs a VBA equivalent
' Target.Offset(0, 2).FormulaR1C1 = "=IF(RC[-1]>0,SUMPRODUCT(-(R7C7:R35000C7<0)*0.5,--(R7C3:R35000C3=RC[-5]))+SUMPRODUCT(--(R7C7:RC7>0),--(R7C3:RC3=RC[-5]),R7C7:RC7),0)"
End If
If Not Intersect(Target, Range(Range("rSurname").Offset(1, 0), Range("rSurname"). _
Offset(UsedRange.Rows.Count + 1, 0))) Is Nothing Then
Target.Offset(0, 1) = ActiveCell.Offset(0, -2) & " " & ActiveCell.Offset(0, -1)
End If
End Sub