Compression Ratio and Boost Query

Never heard of a 6psi limit but even that's good for a boosted hp of about 1.4 x NA hp.

That's probably consider an rule of thumb boost level on pump gas without an intercooler.

There's things you can do to raise that, but at least you can turn boost up or down as required then find ways to keep turning it up, water/meth, intercooler etc..