Notice how the coil opening is centered on the screws. This is a 2.5mm ID that seems to be the sweet spot.
Plenty of cotton running through the coil enough for gradual resistance but not enough that your cotton is pulling apart as your pulling it through the coil.
Tails go up through the coil housing like bunny ears and then the housing is slid over them and secured.
I then pull them to the side and trim leaving approximately an 1/8 inch tail left on each side.
Using a small flat head or equivalent i gently poke the tails down on each side no stuffing them tightly but ensuring that it touches the bottom of the build deck. I also inspect and make sure that none of the cotton is wildly poking down the air hole under the coil. this will cause leaking.
As you turn the housing on its side you should be able to see just enough poof of cotton poking out of the juice channel holes. This means you have enough cotton in your device.
The only way you can shut the air off on the top tank mini is in between the small hole and medium hole...there is no true "all the way off" setting so you will just have to play with it.
Ensure you leave your slight air bubble at the top for proper juice flow then close the top and open your afc back up. You should have no issues with leaking if these steps are followed. Leave the tank on its side for a prolonged period of time and you may have flooding and gurgling issues but still should not have leaking...i tend to go to sleep with my Mod and Subtank mini....I place it on its side right before i go to sleep and havent had a leak yet in over 3 months.