Copy formula and paste same row to last column used

Hello,

I have been struggling with finding the last used column and pasting a formula in the same row to the last column. Here is what I have used and it isn't working.:

Sub Subtotal()

    Dim LastRow As Long
    Dim LastCol As Long
    Dim lastcell As String

   'Define Variables
    LastRow = Range("A" & Rows.Count).End(xlUp).Row
    ActiveSheet.UsedRange.Select
    LastCol = Range("A" & Columns.Count).End(xlToLeft).Column
    lastcell = ActiveCell.SpecialCells(xlLastCell).Address
    iLastColumn = Range("A7").End(xlToRight).Row
    a = Cells(7, Columns.Count).End(xlToLeft).Column

    Range("R1").Select
    Selection.NumberFormat = "#,##0"
    ActiveCell.FormulaR1C1 = "=SUBTOTAL(9,R[2]C:R[" & LastRow & "]C)"
    Range("R1").Select
    Selection.Copy
'it errors out at next line, I just want to take the ActiveCell.Formula above and copy it all the way to the last column used.
    Range("R1:" & iLastColumn).Select
    ActiveSheet.Paste
    Application.CutCopyMode = False

Any thoughts?

Many thanks!
Matt