calculate tax and simulate in miro

Hi All We are doing one project  that is  invoice application here we are getting regarding simulate  button while posting invoice through miro there is simulate button is there same button we are planning to put in our application But we are getting confuse from where to take the data from which table and how please help We want calculate tax check how its is in miro same kind check we want to add in our application how to take the data and from which table please help us we have already tried in all way but we are failed please help