420A 440cc injectors base fuel pressure? Vacuum leak?

[Brad Bell]

Just installed a set of 440cc injectors on my turbo 420a. They seem to rungreat but I am confused as to what base fuel pressure I should have my SFMU set to, and at what ratio should the fuel pressure should be increasing? I have it set to 20 psi right now and got it to idle pretty decent, at around 14.6-15.0 AFR. Not sure if this is too high or too low. Also the car backfires quite a bit on deceleration if still in gear. I thought this may be because of a vacuum leak because I didn't have this problem before, installing the injectors, boost gauge reads around -18inHg which seems normal. Although when coming to a stop sometimes the gauge will drop to around -15inHg and the car will almost stall out, so I'm not sure . I'm hoping someone with a similar setup can help me out with this, thanks!