I would put service tax value in app.config (if windows application) or web.config(if asp.net application).
Then read this value in the code prior to calculation as below
if(ConfigurationManager.AppSettings.Get("ServiceTax") != null)
{
double serviceTax = Convert.ToDouble(ConfigurationManager.AppSettings.Get("ServiceTax"));
txt_servicetax.Text = Math.Round((o + p) * serviceTax).ToString();
}
Hope that helps.
Milind