Any build with a resistance of .6-.9 OHms is recommended but I like .4-.7 myself.
Yeah, that's about where I like to be at with it, too.
Claptons are the best way to go. Gives all of that heat somewhere to go. It also keeps the wicking problems at bay. High-powered, fast-heating, self-wicking, high-surface area coils are what series does best. The wicking on standard coils can really hold you back in series. If you want full flavor with your hard-hitting vapor, that's the way to go. Personally, I like dual 38/28 between .6-.7 or dual 38/26 at around .55. Not the cloudiest vapes, but the flavor, speed, and texture are off the wall. With those, you can go as low as 2mm for your diameter.
I mean, you can do standard coils, too... ...you just need to keep the diameter at a minimum of 1/8". I've got some fierce 9/64" .37 dual 23's in my Mutation X V3 that can really toss some shit around, but it kills the flavor a good bit. You can go even lower in gauge if you want, but they're gonna reach a point where they're either gonna be huge/slow or generating stupid-high wattage. Assuming a voltage drop to 7.6 (which in my limited testing seems nominal for low-res builds with 25r's,) a .3 is yielding just short of 200w.