Your vehicle's OE fuel pressure may be 58 psi...but that is irrelevant with an upgraded fuel system that uses an entirely different regulation strategy. (especially if it has added forced induction)
In the context of port injection, base fuel pressure is the pressure difference between the inlet and outlet of the injectors (often referred to as ΔP or "delta P"). The inlet of fuel injectors is the rail pressure, and the outlet of the injectors is the intake pressure (iow, boost or vacuum). When you set the "base pressure" on the regulator with either the vacuum line disconnected or the engine off, this is ΔP. Note that fuel rail pressure will drop when you apply engine vacuum to the MAP reference port on the regulator...this is to maintain constant ΔP since the outlet of the injectors (inlet) is under negative pressure (vacuum). Furthermore, you'll notice that fuel pressure will increase (from the base pressure) based on positive intake pressure (boost).
We recommend the lowest possible base fuel pressure to get a proper spray pattern out of the injectors. Bosch injectors need 43 psi ΔP minimum to achieve a proper spray pattern. If the base pressure is set higher than 43psi, the injectors behave larger while making the pumps work harder, resulting in less flow, more fuel heat, reduced pump lifespan, and accelerated corrosive effects of E85 additives.
The base fuel pressure must be set without engine vacuum applied to the regulator, and we recommend doing this with the engine hot. You can either do this with the engine off (and pump running) or by disconnecting the vacuum line from the regulator while the engine is idling. Adjust the pressure adjustment screw on the regulator so that the pressure reads the desired base pressure (ie. 43 psi) and lock the jam nut. When you apply engine vacuum to the regulator, the pressure will drop by the engine vacuum. So, if your engine vacuum is 16 inHg (same as 8 psi), the fuel pressure will drop to 35 psi. Do not re-adjust the regulator! Also, expect the pressure to be a little higher when you start up cold. This is also normal and should not be re-adjusted.