When seeing 500fps regardless of hammer settings your pressure was too low, when it was rising with every shot and dropping back when refilled it was too high, so the springs etc that you had in at that time are capable of doing the job, you need to set it somewhere between those 2 settings and see how it behaves then diagnose whether it is right, high or low, you will get there!
When the power was rising with each shot, this is because the pressure behind the valve is too great for the hammer to knock it open fully, as the pressure acting on the back of the valve comes down the hammer knocks the valve open further increasing power, refilling the res increases the pressure behind the valve again. If you understand how to diagnose the symptoms finding the best setting even without actually knowing the number of bar is not difficult, a little time consuming maybe.
HFT, for people with only 2 buttucks!
VooDoo TwoTwo The next chapter.