I generally start by trying to find out what the maximum power is with the pellet I think I want to use, and how far I can back off the hammer spring preload before the velocity starts to drop (the "knee" of the velocity vs. preload curve).... This is only because I am curious, and have a tendency to tune for about as much power as I can obtain efficiently.... If you have a given power level (FPE) in mind, that part of the process is a waste of time (although you may learn something, never a bad thing)....
If I am tuning for power, using a given fill pressure and pellet weight, I start at the "knee" (no sense going higher, its very wasteful of air, and results in a Korean Cliff style of tune).... I then back off the preload until I get a typical bell-curve, where the first half of the curve rises 4% of the velocity at the peak, or slightly less, and stop shooting when the velocity falls back 4% below the peak.... This gives me a 4% ES, starting at my fill pressure, and giving the most shots I can get within that ES.... Since a 4% ES is good to about 50 yards, that is generally where I stop, as that covers most of the shooting I would do with an unregulated PCP.... If I happen to be tuning an unregulated PCP for longer range, I use a tighter ES.... 3% for 75 yards, and 2% maximum for 100 yards....
With a regulated gun, I expect less than a 2% ES always, if I don't get it, there is something wrong.... Usually the ES with unsorted pellets is a whisker over 1%.... Since I seldom shoot over 100 yards, tuning a regulated gun becomes a matter of adjusting regulator setpoint pressure and hammer spring preload until I get the FPE level I want with the greatest shot count.... This inevitably occurs when the gun is tuned just below the "knee" of the curve, ie below the plateau and above the downslope, or right at the top of the downslope.... Any major change in pellet weight will change the tune, of course, whether regulated or not....
After I have achieved the tune I want, I then double-check all suitable pellets.... If I didn't guess right the first time, and find something else is more accurate.... I fine tune using the most accurate pellet I have for the intended job.... 95% of the time that ends up being some weight of JSB Exact....
If I want significantly less power than the gun is capable of, then I either back off the hammer spring preload and tune for a lower fill pressure (or regulator setpoint).... or install a smaller transfer port.... or both.... Chokng up on the transfer port allows you to maintain the original fill pressure while reducing the FPE level, something you can't do with hammer spring preload.... because less power is then accompanied by lower operating pressures....
Bob