Get Tax value for Purchline

 purchTotals purchTotals = PurchTotals::newPurchTable(PurchTable::find('000869'));
 purchTotals.calc();

 purchTotals.tax().sourceSingleLine(true, true);
 taxAmountCur    = purchTotals.tax().totalTaxAmountSingleLine(purchLine.TableId, purchLine.RecId, true, false);

// or from TaxJournalTrans
Select TaxJournalTrans where TaxJournalTrans.InventTransId == purchLine.InventTransId;