Is there a basic rule of thumb for boost to power?
I.E. 10psi on top of 500hp = 75hp?

Have a 383 at around 300--325hp and wonder ROUGHLY have much boost it would take to achieve 400hp?

Many will argue the idea.... but I go by the rule of thumb that 15 psi of boost will double your NA power output. It's in no way set in stone, but has been pretty much right on for everything I've ever boosted. So if you are at a true 300 hp - you will need 5psi of boost to get to 400.

It also takes power to make power, ie backpressure spooling a turbo or a belt spinning a supercharger. Different units consume different amounts of power and then there's the heat output issue as well. In theory 15psi should double your HP because that's 2x the air intake over sea level atmospheric pressure. In the real-world that's not always the case however.