Hi,

With this code I get the expected result ...

Code:
Sub Test_A()
    Dim Riga As Long
    Dim Dn As Long
    Dim rng As Variant
    Dim Num As Integer
    Num = Cells(2, 12)              'Value = 10
    Dn = Range("C" & Rows.Count).End(xlUp).Row
    rng = Range("IV1:IV" & Dn)      'Range without any data
    For Riga = Num + 1 To UBound(rng)
        rng(Riga, 1) = Evaluate("SUM(C" & Riga - Num + 1 & ":" & "C" & Riga & ")/L2")   '=SUM(C2:C11)/L2
    Next
    Range("O1:O" & Dn) = rng
End Sub
I used an array to avoid delays with 500,000 rows to update.
Definitely worth a general overhaul to improve the code.