How did a high speed adjustment thread for the 390XP turn into a "towel hanging contest"?.....LOL.
Tuning for maximum no-load rpms is NOT how to tune a saw, especially if it has a rev-limiting coil in it. The coil will mimic the 4-stroking caused by a close "H" speed carb setting.
I much prefer to start out just a tad "rich" with the "H" speed screw with noticeable 4 stroking a tad early in the rpm range, then start making cuts and leaning the saw up every so slightly with each cut until it makes the best power. With each cut made remove the load about half way thru the cut and make sure it 4-strokes nicely then cleans up instantly when the load is applied.
With "H" speed tuning, unless you are in some sort of speed cutting contest it is best to er a tad rich than lean. This brings in more lubricant for the engine and reduces piston speed and EGT's, all for the better of the saw in the big scheme of things......IMHO.....Cliff