You may want to try:
Dim lRow as Long
lRow = Cells(Rows.Count, "A").End(xlUp).Row + 2
This will give you the cell 2 rows below the last used row.
Then you can use lRow to target your desired cell and set its '.Formula' :)
I was just poking around and don't mean to revive a dead thread but the following code may help with exporting to Excel:
DoCmd.OutputTo _
ObjectType:=acOutputQuery, _
ObjectName:="Query or Report Name", _
OutputFormat:=acFormatXLSX, _...