I'm afraid you've asked the "1bar mong" question so don't expect a serious answer from most people. It's a bit like asking for a skyhook or a long stand.
I'll give you a break, standard is 0.7bar so Nissan said 20 years ago that it was "safe".
Many people run 1.0bar and it works well most of the time.
Some people run 1.2bar and it still works though 1.2bar sometimes opens a can of worms with standard MAF capability.
Boost pressure doesn't actually tell you anything though as your T28 will run out of air past 330hp give or take. A bigger turbo will take a standard engine to >400hp with the right mapping.
However, running more boost will only exacerbate problems you already have and you might have a piston fail. Equally you might have the clutch fail or just a PITA misfire to solve.
No one can tell you what is safest max/optimum hence the pi** takes
If you stick it on a RR you can first check your car is running right now. When we've done a Group RR and around 20% of the cars found out they had an issue. You can set your car to 1bar and check again, if you're feeling brave go to 1.1bar or even 1.2bar but you might find you start to hit problems. Also don't complain if in 6 months time the extra power has accelerated the demise of your engine/clutch. For safety less is more with boost.
Welcome to the club