Your port timings will be way different from what you want to do with .015. If you plan on doing a 4mil have the cylinders ported for that by a sponsor on here. As far as pipes I take it you are trying to build your own? I would get into contact with someone like Shearer or Jim at Passion. They have plenty of pipe experience. I wouldn't recommend a 2 into 1 carb setup with a 4 mil either.
Where you are looking at the plating it is on the flat ring end of the plug. All plugs are plated in nickel or zinc I believe. Anyhow the ground strap is welded to that ring. As you get you fuel mixture right the plating will start to burn off of the end of the plug.
Notice in this picture how there is roughly 40% of the plating still on the end of the plug. A safe tune is around 25% left. I shoot for around 5% left for my drag bikes.
That needle is leaner than an E needle. The further towards the end of the alphabet you go, the richer. Start with your power jets wide open, which is the screw is backed all the way out. On alky you aren't looking for the color as you are more looking at the plating on the plug and the strap. The plating will tell you where your fuel is at the strap will tell you about your timing.
I would suggest running possibly an EGN or richer down there. I'm running the EGN at the middle position up here. You do have power jets on those carbs?
I would say ditch those stock carbs and snag up some 35mm PWKS. That's the first thing I do when someone brings a bike to me, whether it is stock porting or not. That serval will love more air.
20cc domes may be a little much for pump gas at your altitude. Most guys stick around 22cc to 23cc for pump fuel. As for a place to start on jetting 350 mains and 30 pilots with 2 turns on your screws? I'm at 2700 so... Start with fresh plugs and stay fat then jet it down till it cleans up on the top end for main jetting.
I have a bike up here at 2200 feet with nearly the same mods. I has the large K&N single no lid, FMFs, decked head, and some mild porting. It is at 320 mains, 27.5 pilots. It's pretty close to where it should be.
Already picked up an axle and stator. Thanks. Still need:
Billet Stator Cover
Arms non drag
Carrier with brake stay
Case saver
Flywheel
Intakes for 40mm carbs