Hello Jon,
Try this...
With Range("S10", Cells(rows.Count, "S").End(xlUp))
.FormulaR1C1 = "=RC[-4]*RC[-11]"
.NumberFormat = "#,##0.00"
End With
Note if S10 is empty then the first cell could be from S1 to S9.
To be sure your first cell is S10 use this...
Dim RngBeg As Range
Dim RngEnd As Range
Set RngBeg = Range("S10")
Set RngEnd = Cells(Rows.Count, "S").End(xlUp)
If RngEnd.Row < RngBeg.Row Then Set RngEnd = RngBeg
With Range(RngBEg, RngEnd)
.FormulaR1C1 = "=RC[-4]*RC[-11]"
.NumberFormat = "#,##0.00
End With