The BEST way is to try a couple of shots at EVERY preload setting and graph the results like this....but you are correct, starting from maximum and decreasing the preload until the velocity drops about 3-5% below the plateau is the quick way to get there....
That gun's best tune was 4-5 turns out on the "knee" of the curve.... Adding preload once the velocity plateaus only wastes air.... Dropping down onto the downslope by reducing the preload will give more shots and great efficiency (at lower FPE), but when the pressure falls below the setpoint the velocity will spike UP.... I tune all my regulated PCPs to the knee, and if that doesn't meet my needs, then I change the setpoint, and repeat the process....
You only need to fire a shot or two at each preload setting to find the knee.... Once you find it, then a string or two with small adjustments, within that knee, will find you the balance between FPE and shot count you want.... For example, on the above I might have shot a string at 4 turns, 4.5 turns, and 5 turns, before settling on 4.5 turns out from coil bind on the preload.... with this result....
Bob
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