You know that black box in the enginebay on the firewall on the drivers side, thats the MAP sensor that feeds the triple gauge boost reading, I tee'd in there as the pressure source for the PSI reading. You can tee off at the FPR directly, it will technically be less laggy than tee'ing into what I used. The stock MAP sensor gets pressure reading from the FPR anyway, it just enters the balance tube first, and a hose comes out the other end to it, then connects to that black box.
With the engine pressure source (stock one is the bled source), I can't remember if mine is before or after the throttle bodies.