Tax Percentage value

 TmpTaxWorkTrans tmpTax;
 purchTotals purchTotals = PurchTotals::newPurchTable(PurchTable);
 purchTotals.calc();
 tmpTax.setTmpData(purchTotals.tax().tmpTaxWorkTrans());
 select firstonly tmpTax;
 real TaxValue = TaxData::percent(tmpTax.TaxCode,today(),0);
 Info(strFmt("%1",TaxValue));
 TaxValue = TaxData::find(tmpTax.TaxCode, Systemdateget(), 0).TaxValue;
 Info(strFmt("%1",TaxValue));
 NW_CertificationOfCompletionHeader.TaxValue          = TaxValue;

https://mycsharpdeveloper.wordpress.com/2015/07/03/how-to-get-salespurchase-taxgstvat-info-for-report/

// OR
 Line.TaxValue = strFmt("%1%",this.getTaxPercent(PurchLine.TaxGroup, PurchLine.TaxItemGroup));

public TaxValue getTaxPercent(TaxGroup _taxGroup, TaxItemGroup _taxItemGroup)
{
    TaxGroupData    taxGroupData;
    TaxOnItem       taxOnItem;
    TaxData         taxData;
    select firstonly TaxCode from taxGroupData
    index hint TaxGroupIdx
        where taxGroupData.TaxGroup == _taxGroup
        join taxOnItem
        where taxOnItem.TaxItemGroup == _taxItemGroup
        && taxOnItem.TaxCode == taxGroupData.TaxCode;
    select firstonly TaxValue from taxData
        where taxData.TaxCode == taxGroupData.TaxCode;
    //&& taxData.TaxFromDate <= systemDateGet() &&  taxData.TaxToDate >= systemDateGet();
    return taxData.TaxValue;
}