You can lower pop pressure by sanding/lapping the spring and zero in very precisely on a pop pressure. That method requires that one set all of the injectors slightly above the desired pressure and then drop them down to the exact pressure desired. I have adjusted injectors to within 10 psi on a set of four. I usually opt for 30 psi as closer than that takes a whole lot more time and I cannot notice a difference in performance. I also opt for the high spec on pop pressures and the advanced spec for timing as that combo assists economy and cold starts due to bettter atomization.

Andrew