I typically build 3mm ID, mostly preference, but for most tanks it seems to give a decent size/density for wicking with minimal trimming. 2.5mm or a larger 3.5, or maybe even 4mm would probably be good in the MM as well. I've found for for most juice I vape 3 seems to be optimal. Do experiment though, and find your own preference..
Sorry, not a huge TC person, I do use it, just not on everything If I were going to run TC on the Merlin with .24g SS I'd probably try 6, or maybe 8 wraps on a 3, or 3.5mm coil. Many variables come into play, and as mentioned, even your preferred flavor can effect your build preference. Best thing you can do is experiment and see what you like. You may even want to try come Stainless exotic coils. Or maybe some triple twisted .28 430 they usually work quite nicely in this type of tank.
