I've drilled the outlet air holes to ~2.7mm to match the inlets and like to build the coils so they are just slightly wider than the airholes. On a mech I target 0.7Ω, as this gives me the temperature of vapour I like in the 20-25 watt range over the course of the battery's discharge (3.8-4.2v).
My final build is always the same - dual 1.4Ω coils (0.7Ω effective) with 27ga Kanthal, exactly 3mm in diameter, spaced exactly 1mm above the holes (using a 1mm feeler) and parallel to the posts. Legs are 4mm total length, 3mm to pin. Wicked with 3.5mm strips of Japanese cotton, either Shiseido or Koh Gen Do.
This is my preferred setup for flavour. Once I dialed this in, I premade about 20 coils, trimmed, level, primed (fired and 'squeezed') and ready to install, so swaps are simple and quick.
I tried a single coil, and flavour was good, but the vapour wasn't satisfying. I also run the Magma without the air control ring entirely. With the widened air outlets, this has eliminated whistling and allows for a very smooth draw.
Ha! I have worn the Magma rings as rings (just for kicks), but other than that I got no use for them.