It depends on how u drive it. Even 20+ psi can be safe. But 18psi is the baseline imho. Just be minded, boost is not the only guideline, so is timing. U can run high boost with conservative timing and still safe.

Check with urself:
Do I redline it consecutively?
Do I boot it after sitting in a jam?
Do I push it when weather is really hot?
Do I use full boost for the first 10min from cold start?
Do I use good 98 fuel and not use up the tank?

I find the above situation cause knocking(10x higher combustion chamber pressure) which u dont want at high boost.

Good luck with ur tuning.

using the Perth-WRX mobile app

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MY08 STI 422whp at 1.5bar on E85, built by Diversion Garage, assembled and tuned by EvoR

Update from new owner: 375whp at 1.4bar on 98, at Tokyo Motorsport