back in the day it was trial and error, now I use performance trend engine analizer software, it calculates an advace curve for what ever octane fuel you want to run. Like most programs garbage in garbage out, but if you take the time to be accurate with the info it works good. I just did a procharged 410 mopar with a fitech, calculted the advance curve with and with out boost imputed the numbers into the Fitech computer as a starting point for the ignition advance, it ran great. I am running 6 lbs of boost on pump gas. When I decide to change pulleys and good gas, will calculate again and see what happens.