how to write app.config file code help me

Question

i design application using c sharp.
in that i have threecolumn as follows.
Rental cost ,Diesel service, Service Tax.
 

Rental Cost 231270
Diesel ser 19767
Ser Tax 30124(231270 + 19767 * 12/100).
Code as follows;
 
double o = Convert.ToDouble(txt_rentalcost.Text);
double p = Convert.ToDouble(txt_dieselservice.Text);
txt_servicetax.Text = Math.Round((o + p) * 0.12).ToString();
 
i am doing service tax calculation at that time service tax is 12 percentage.suppose in next year service tax percentage is 12.36.
 
In App.config how to write the code.please send the code.first time i am uisng app config file.please send the code for my above question.