You might not be able to get the fuel ring much tighter than that on pump gas. The green on the plug means something...I forget what. I’ll get the link to what shrinker wrote on plug reading and post it here so you can read through it.
Just remember...if you are driving this car on the street (I think you do, I just don’t remember for sure) and you are using a power valve, dropping the primary main jet to clean up WOT may make you lean at a cruise. Of course, the opposite is true...adding primary main jet to clean up a lean WOT can make you fat at a cruise.
I mention that because it happens all the time. If you get to that point of being either lean or rich at cruise, you can correct the primary main jet and then change the power valve channel restricters to get your WOT back in shape.
Here is the link. He gets into what the green means. I should remember it...but ya know...getting old and stuff.
http://gallery.myff.org/gallery/1486369/Sparkplug+reading.pdf
Edit: I forgot to mention that you are doing it correctly in that you are jetting for best MPH. When you get to the point the MPH goes backwards, jet back up a skosh, and maybe add a bit of fuel so when you get out of nose bleed DA’s, you’ll be a bit safer then look at the fuel ring, and that’s where you want it to be for the fuel you are using.