How much cotton do you use?
In terms of thickness, enough so that it pulls through the coil relatively easily, a small amount of resistance. If you pull a used wick out and it doesn't come out easily, you've use too much. However, the rayon is a bit more forgiving here, it shrinks down a little once it soaks up juice. Even in cases where the wick I've made is way too thick, it seems to work just fine.
What's the average length you leave exposed from the top of the vert coil?
I cut the tops off flush with the top of the center post. The idea here is that the wick above the coil acts as a reservoir. When dry, it pulls juice PAST the coil, so that you're getting juice wicked from both ends, not just underneath. I also leave mine touching the center post.
What cotton do you use? (Ive got both KGD and cellu cotton)
Cellucotton (Rayon). I don't find it tastes noticeably different to KGD, but it's way cheaper. I started using cotton, then KGD, then tried Rayon and never felt a need to switch back.
On a single coil do you still cover the second chamber with cotton to soak the excess juice up?
No, I don't do this in any situation, single or dual. I pull the wick down through the vertical coil from underneath, pull it back outside to cut the tail flush, then tuck it back in so it only just touches the bottom of the deck. The rest of the space inside the juice well is open, no cotton. The wick is just a vertical pillar starting from the bottom of the deck, going through the coil, ending flush with the top of the center post.
Is it a good idea to maybe leave some cotton on the center screw so when dripping from the drip tip it hits directly onto the cotton wool or is that just a breathtakingly stupid idea?
Not stupid at all. If you go back a page and check inswva's pics, I build pretty much identical to that - the only difference being my coils are hard into one corner of the deck opening (as opposed to being centered in the opening), and perhaps a little closer in toward the center post. When juice gets dripped down, if it doesn't touch the top of the wick or the coil, it reaches a choke point between the bottom of the coil, bottom of the center post, and the flat area the negative screw sits in.
That said, I still take my top cap off to fill. I stop when I see surface tension forming in juice well, if that makes sense. The juice level is perhaps 0.5 - 1.0mm below the level of the top of the deck. Go much further and it can leak if you tilt, and it also chokes the vapor off a bit until you vape it down a little.