kirt – it depends on what tires and condition of the track.. If its for NJSS on Bstone YLC then start with an 11-76. If on a Bstone YLB, then go to an 11-75. If the chassis is set up right, that should be right were you want to be. If the track is like this weekend and the track is covered in pollen, go up 1 tooth. I only change gear until its within a few tenths of race pace then play with flex lengths.
That is for seniors, no idea where to start an X30 in juniors, only have had junior rotaxs. Hope that helps.