The dwell is the time in milliseconds that the solenoid is activated. During activation air supplied by your LPR is sent to bolt system to move it forward and then back to cycle the marker. A longer (or higher) dwell time means there will be a larger VOLUME of air supplied to the bolt for the cycle. A lower dwell provides a smaller volume of air. To accommodate for a longer dwell time at 21 ms (stock is 18) one can drop the LPR pressure to achieve the same consistent firing rates and hence have a smoother shot because of the lower pressure. A lower dwell time requires a higher operating pressure (just the opposite).
My suggestion, set it to stock at 18, back your dwell out about 4 turns. If the consistency is good and you don't have drop off your gun will shoot paint bullets at a high level for a long time!