the lowest psi you can run is the gate spring, on the RB25 manual tuborcharger from memory it is 7psi, on the auto RB25 turbocharger it is 5psi from memory.
if you want lower boos than this, then you can simply wire the wastegate flap open and youll probably get 1 or 2psi if you are lucky at full load
ECR33 manual is 385mmhg which is 7.4446 psi ECR33 auto is 270mmhg which is 5.220 psi