here is an paragraph taken from http://www.endlesspb.com/tech/efficiency.html
on how to sweet spot your reg:
Start by turning your inline regulator all the way down to zero psi, also set your velocity adjuster at two turns in from all the way out. Now, increase your pressure slowly while shooting the marker over the chronograph. Adjust your pressure 25 psi at a time, and take five shots (give or take) over the chronograph to establish a velocity at that pressure. Continue this process until the velocity peaks, which is when any increase in pressure will result in a decrease of velocity. This point varies for every marker and is the most efficient pressure setting for your spring set. When the velocity drops, start lowering the pressure (5 psi at a time). The velocity should start to go up again. Gently nudge the pressure lower and lower (using those small increments) until any change, be it up or down, results in a lower velocity.