protected function getTaxPercentAttribute()
    {
        $fTaxPercent = $this->getAttribute('tax_percent');
        if ($fTaxPercent === null) {
            $fTaxPercent = TaxHelper::instance()->getTaxPercent($this->tax_list);
            $this->setAttribute('tax_percent', $fTaxPercent);
        }

        return $fTaxPercent;
    }