public function getProportionalVatPercent()
    {
        $dTotalSum = 0;

        foreach ($this->_aList as $oPrice) {
            $dTotalSum += $oPrice->getNettoPrice();
        }

        $dProportionalVat = 0;

        foreach ($this->_aList as $oPrice) {
            if ($dTotalSum > 0) {
                $dProportionalVat += $oPrice->getNettoPrice() / $dTotalSum * $oPrice->getVat();
            }
        }

        return $dProportionalVat;
    }