If xDgr = 0 Then xDgr = xMin

If xDgr > 480 Then xDgr = xMax

galiba bu 2 satırların yerlerini karıştırmışım)

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Set S1 = Sheets("Pergole")

yC8 = S1.Range("C8").Value

xB21 = Int((yC8 + 225) / 450) - 1 '<== eklenen

xB22 = Int((yC8 + 225) / 400) + 1

For xB = xB21 To xB22

    xSon = (yC8 - 175) / (xB - 1)

    If xSon > 450 Then

        xMin = xB

    ElseIf xSon < 400 Then

        xMax = xB

    Else

        xDgr = xB

        Exit For

    End If

Next xB

If xDgr = 0 Then xDgr = xMin

If xDgr > 480 Then xDgr = xMax

S1.Range("B21").Value = xDgr