Stop in for a cup of coffee

Hey Hoppy.. Let me tap your expertise.. I know all the words, initial timing at 12, 34 total advance, all in by 2800 rpm.... But, if I were to send you a distributor for a custom curve, how do I 'know' that I want 12/34/2800 vs 10/36/3200 or some other random collection of numbers? Is there some secret voodoo magic or is it trial and error for any particular engine?