Reference LastCol VB

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??

Maybe like this (untested)?

    With Range("O5").End(xlToRight)
        .Resize(lastrow1 - .Row + 1).FormulaR1C1 = "=RC[-11]-RC[-10]"
    End With

Well, It turns out that I have 2 columns to add formulas. I did some research and was able to set my lastcol variables and likewise add the formulas in the correct spots.
Here is what I have. It seems to work perfect.

Sheets("FinHL").Activate
Columns("E:E").Insert shift:=xlToRight
Range("D3:D200").Copy
Range("E3").PasteSpecial Paste:=xlPasteValues
Application.CutCopyMode = False
Dim lastcol1 As Long
Dim lastcol2 As Long
Dim lastrow1 As Long
Dim lastrow2 As Long
lastcol1 = Cells(5, Columns.Count).End(xlToLeft).Offset(0, -1).Column
lastcol2 = Cells(5, Columns.Count).End(xlToLeft).Column
lastrow1 = Range("B5").End(xlDown).Row
lastrow2 = Range("B16").End(xlDown).Row
Range(Cells(5, lastcol1), Cells(5, lastcol1)).FormulaR1C1 = "=RC4-RC5"
Range(Cells(5, lastcol1), Cells(lastrow1, lastcol1)).FillDown
Range(Cells(16, lastcol1), Cells(16, lastcol1)).FormulaR1C1 = "=RC4-RC5"
Range(Cells(16, lastcol1), Cells(lastrow2, lastcol1)).FillDown
Range(Cells(5, lastcol2), Cells(5, lastcol2)).FormulaR1C1 = "=((RC4-RC5)/RC5)"
Range(Cells(5, lastcol2), Cells(lastrow1, lastcol2)).FillDown
Range(Cells(16, lastcol2), Cells(16, lastcol2)).FormulaR1C1 = "=((RC4-RC5)/RC5)"
Range(Cells(16, lastcol2), Cells(lastrow2, lastcol2)).FillDown
Range(Cells(lastrow2, lastcol1), Cells(lastrow2, lastcol2)).Borders(xlEdgeBottom).LineStyle = xlContinuous
Range("A1").Activate