The point of a boost gauge is to measure the pressure difference between atmospheric and inside the manifold, so 0psi or -5psi can be correct
As for how far the standard map goes, I'd say it can read up to a specific volume of air.
So what would be say 5psi at 3000rpm = 4.1 volt signal on the AFM
would be the same max if you had say 3psi at 8000rpm and 4.1 volt afm signal.
If you look at the powerfc maps, the standard map has areas mapped out that you would NEVER reach, like near maximum airflow meter voltages at 1500rpm. Would never get there unless you had a crap load of boost, haha.