Excelde Faiz Hesaplama

Sub hesapla()
k = 4
Do While Cells(k, 7) <> ""
    tutar = Cells(k, 7)
    bastar = Cells(k, 8)
    sontar = Cells(k, 9)
    tmpbastar = bastar
    j = 4
    faiz = 0
    devam = True
    Do Until devam = False
  faiztar = Cells(j, 3)
  If Cells(j, 3) = "" Or faiztar > sontar Then faiztar = sontar: devam = False
  If bastar > faiztar Then GoTo 20
faizor = Cells(j - 1, 4)
faiz = faiz + ((faiztar - tmpbastar) * tutar * faizor) / 36500
tmpbastar = faiztar
20
j = j + 1
    Loop
    Cells(k, 10) = faiz
    k = k + 1: faiz = 0
Loop
End Sub