XElement xTotalTax = new XElement("totalTax");
XElement xInvoiceDiscountPercent = new XElement("invoiceDiscountPercent");
decimal dAmtIncVat = decimal.Parse(value.Element("amountIncludingVAT").Value);
decimal damount = decimal.Parse(value.Element("amount").Value);
decimal dInvDisAmt = decimal.Parse(value.Element("invoiceDiscountAmount").Value);
decimal dTotalTax = dAmtIncVat - damount;
xTotalTax.Value = dTotalTax.ToString();
decimal dInvDisPer = Math.Round(dInvDisAmt * 100 / (dInvDisAmt + damount), 2);
xInvoiceDiscountPercent.Value = dInvDisPer.ToString();
xHeader.Add(xTotalTax);
xHeader.Add(xInvoiceDiscountPercent);
requirements: add element for xHeader.
first step is to create new xEkement, e.g "totalTax", then set the value for element "totalTax", finally, add element.