use a number 6 plug for normal conditions (15 to 30 degrees celc +) tune your low end on the bench so it can idle "nicely" and consistantly. basicly so that it can idle for 15 to 20 sec and you can stil WOT it without it dieing or bogging down excessivley.
then on the track do a few laps as fast as you can without crashing and lean it according to top end. you should tune it so that it DOESN'T deteriorate in performance during a race.