This should be in the cocker forum, but as for 3-way timing, that is elementary cocker stuff that you will learn in time and become comfortable with.
If your o-rings are in good condition, and are oiled properly, you likely need to adjust the collar (with the set screws) on the actuator rod that holds the 3-way plunger until the leak stops. Either clockwise or counter-clockwise depending upon where it is set now. Keep pulling the trigger until it is cycling properly and set yourself on a setting that seems to be working well. If you turned it too far one way, just work your way back. Once you have no burps, hiccups etc. you are in the ballpark. Then what I sometimes do is place a tissue over the feed tube. When I pull the trigger if it sucks the tissue into the tube and rips it or sucks it slightly in, I leave it. People will tell you this is suction or tissue timing. I check it just to make sure I have no significant blowback.
Check out the many threads on timing. Timing is something you need to learn because you may need to adjust it from time to time at times when it is less than optimal.
As for the pneumatic reg, adjust the pressure to its lowest setting so the gun won't cock and slowly turn it up until the gun does cock, then add a little (say an 1/8th or 1/4 turn) as a safety in case off reg drop off etc. That is the easiest way to do it. As for a baseline PSI I would say just adjust it until the gun operates properly since most people do not have a gauge to measure the PSI mounted on their guns.