I need to edit the following code so the formula does not go into "O"5 and "O"16, but into the last used col 5 and 16, then filldown.

Dim lastrow1 As Long
Dim lastrow2 As Long

lastrow1 = Range("B5").End(xlDown).Row
lastrow2 = Range("B16").End(xlDown).Row

    Range("O5").FormulaR1C1 = "=RC[-11]-RC[-10]"
    Range("O5:O" & lastrow1).FillDown
    
    Range("O16").FormulaR1C1 = "=RC[-11]-RC[-10]"
    Range("O16:O" & lastrow2).FillDown
Any ideas??