VBA formula with dynamic reference

I have this code filling in blank cells in column L with a formula.
The issue is, the sum(O2:R2) as it needs to use the row number it is currently in.
If the first blank in column L is row 5, this formula will not work.

Can anyone help please!?? Thank you!

Sub urtreatmentcompleted()
    Application.Calculation = xlManual
    Dim ws As Worksheet
    Set ws = ThisWorkbook.Worksheets("ur data") 'define which worksheet

    Dim LastRow As Long
    LastRow = ws.Cells(ws.Rows.Count, "A").End(xlUp).row

    Dim iRow As Long
    For iRow = LastRow To 1 Step -1
        If ws.Cells(iRow, "L").Value = vbNullString Then
            ws.Rows(iRow).Value = "=if(sum(O2:R2)>=10,""Completed treatment"","""""
        End If
    Next iRow
Application.Calculation = xlAutomatic
End Sub

Select allOpen in new window