For what it's worth to an old thread;

From what we've worked out while trying to understand the MFi system best as we can, the fuel pressure reg vented to atmos is a crude form or barometric correction to the 'base' fuel pressure.

As we know the rise in pressure is taken care of by the ecu and the fuel pressure adjuster fitted to the distribution head. The base pressure it rises from is set by the baro correction.

I like to think of the electronic adjuster as more of a flow destructor than pressure adjuster. Easy way to test is to put fuel pressure gauge at each point in the fuel lines around dist head and suck/blow a tube pushed over the nipple on the pressure reg.