Does ProSeries not have worksheets that show the computations for this situation?  Lacerte does, so all you have to do is drill down to that worksheet to follow the math.  Of course, understanding the concept first "is" rather important.

@MGC94  I know that it was suggested to you that you obtain some basic research material (The Tax Book, or Quickfinder).  BOTH of these have tons of info on the questions you ask; and that's where you should look first for the basics.