How much boost?

[NismoRacer1]

I am swapping for a S13 blacktop with 30k miles on it from a 98 180SX. I've got a new fule pump, FMIC, downpipe, BOV, EBC, all going in in about two weeks. My friend is tuning it for me and wants to tune it for 13psi, but he dynoed his car when he was runnig the T25 at 15psi, and thats what I've read is the max efficeint boost for the T25. What should I tune it at for max performance without damaging the turbo?

Thanks