Calculating totals Amounts of a transaction in window form

decimal discount, shopcost;

if (discounttxt.Text != "" && chk_DiscPerct.Checked==true)
{
discount = (_ttl * Convert.ToDecimal(discounttxt.Text)) / 100;

}
else if (discounttxt.Text != "")
{
discount = Convert.ToDecimal(discounttxt.Text);
}
else { discount = 0; }

if (addcosttxt.Text != "")
{
shopcost = Convert.ToDecimal(addcosttxt.Text);
}
else { shopcost = 0; }
txtsubttotal.Text = (_ttl + shopcost).ToString();
txtNewTotal.Text = (Convert.ToDecimal(txtsubttotal.Text) - discount).ToString();