Find tax value from tax item code and tax group code

//You have to pass these values to the functions whether it comes from PurchLine or SalesLine or any other table.

//Store the result in variable

Tax::calcTaxAmount(purchLine.TaxGroup, purchLine.TaxItemGroup, Systemdateget(), purchLine.CurrencyCode, purchLine.LineAmount, TaxModuleType::Purch);

You have to pass these values to the functions, whether they come from SalesLine or PurchLine