I see a lot of builds with 'shoulders' - right where the wick exits the coil it expands, like the wick is a larger diameter than the coil. I can pretty much guarantee I could use less than half of that & make a much, much better wick out of it. When I thread a wick through, it's loose. When you prime it with liquid, the cotton will swell a bit - I aim to have the wick expand just a tiny, tiny, tiny bit shy of the inside of the coil. It doesn't touch all the way around wet, but you'd have to look pretty darn close to tell that it wasn't.
As for the tails, it's just something you can develop an eye for, knowing how long to cut them. I'd say mine extend maybe 1/4 to 1/3 into the threading below the bottom of the well (before the chamber is put on, of course). I'm very careful not to disturb the ends after they're cut to length. Think mad scientist hair. I gently pinch the ends of the coil where the wick comes out to point them skyward without actually touching the ends with my fingers. You can get the chamber on without disturbing them that way. Then I use an open safety pin to tuck them into place. The point does a good job of directing the fibers where they need to go, but it doesn't compress them or bunch them up.
Getting a super, super fluffy wick with tails of the correct length is key. The diameter is sorta, kinda important, but you can fudge it a little bit without too much consequence.
And again, don't twist it. You want the fibers all going the same way, not in a helical/spiral pattern. Fiber wicks work with surface tension - the fiber itself doesn't wick, the air between the fibers does the work. Twisting interrupts that pattern & chokes the wick off.
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