How to Calculate VAT and Service Tax in C#

public void vat()
{
   decimal pri = Convert.ToDecimal(txt_AMT.Text);
   decimal Vat = Convert.ToDecimal(txt_Vat.Text);
   decimal stax = Convert.ToDecimal(txt_stax.Text);
   decimal servicetaxpay = pri * (stax / 100);//only stax
   decimal totalp = pri + servicetaxpay;//
   decimal vatpay = pri * (Vat / 100);
   decimal totalPI = pri + vatpay;
   decimal totalPIs = pri + servicetaxpay;
   lblvatAmt.Text = " % " + " =" + vatpay;// ("Vat@" + vatpay);
   lblstaxamount.Text = " % " + " =" + servicetaxpay;// ("S.Tax@" + servicetaxpay);
   decimal result1 = vatpay;
   decimal result2 = servicetaxpay;
   lblsvtax.Text = (result1 + result2).ToString();
   txttotalamt.Text = pri+result1+result2 +"Rs.";
   ////+"stax="+stax+"%"//add
   ////+ "StaxPay=" + servicetaxpay//add
   //+ "Stax=" + servicetaxpay
   //+ " vatpay =" + vatpay
   //+ "Total(Include Vat Amount) =" + totalPI);
}