I recently changed the plugs in my 07 Honda Goldwing and after reading lots of material and articles about which plug was considered better I went with an NGK Iridium and followed all the technique data I was able to find, including the use of a small amount of anti seize compound on the threads and torquing to 13 ft.lbs. And using a small amount of dielectric grease in the plug boot. The bike never ran better! But I was curious about the spark gap. My question is, when a gap states a range between .038 and .043 what is the optimal? Or is there a way to determine what optimum gap to use? I set mine at .040 just guessing somewhere in the middle was sufficient. But I was curious if there wasn't some method for determining what the optimum gap should be??
Nobody gaps plugs anymore, unless they are doing something custom for a racing vehicle or similar.
I certainly wouldn't risk damaging an electrode on an NGK Iridium at 10-15 bucks each. The correct plug will come with an optimal gap already factory set. NGK knows far more about what makes their plugs work well than I will ever know.
In general, a wider gap gives a larger spark bolus. But the available electrical energy ultimately decides how much gap is possible, and what can be maintained at high RPM.
I've increased plug gaps on my F Production '72 BMW 2002tii, but that's combined with a very heavy duty Crane/MSD electronic ignition setup, with a slot wheel and optical trigger in place of points. And if I could easily find the exact specification of spark plug I wanted off-the-shelf, I wouldn't bother.
I really wouldn't sweat 5 thousandths of an inch gap range. You already made likely the best choice with NGK Iridium or similar. Money, but well spent. I suspect they will last 50-75K miles minimum.