you are basically forced to start with the stock plug the manufacturer suggested and work up or down in plug heat range due to indications you see in how the current plugs look after being run, now obviously if your radically changing the operational parameters, and your boosting compression, rpms and using nitrous for an example, you can make an educated guess as to if its likely to require a step colder plug, as a start point but the process remains consistent in that you look at the heat indicators like the heat band on the ground electrode and crap on the insulator porcelain, and ash on the plug. but your still going to need to run, inspect and make changes in the ignition advance curve and fuel air ratios and ideally use a fuel/air ratio meter and infrared temp gun in addition to closely inspecting the spark plugs for indications of operational conditions.
