here some information that might be helpful
its basically trial and error
pick a % for the set boost, say 40%, for gain i would start with zero, 1st just try to get the boost level you want, then you can increase the gain to allow it to come onto boost quicker, and work out were the increase in gain = to boost spike.
i don't have start boost on my ebc but from my understand it is when the ebc starts to open the wastegate, it also looks like the closer the start boost is to your set boost, the quicker the car will come onto boost but the more likely you will see boost spike.
i would say start with a lower number then increase it once you have the set boost correct.
also cold weather affects boost and makes it more likly to spike so, if you set it up at night it is more likly to run at a lower level during the day (mine usually spike by only 1-2psi on a cold night)
hope that helps!