Really short answer. .4 ohm, 13.5 wraps on 4mm mandrel, 24g 316l dual contact macro coil.
Short answer. Unless you wear big boy pants and have a good understanding of electronics, batteries, discharge ratings, and ohms law, you shouldn't go below say .35 ohm or .4 ohm. You'll get plenty of power and definitely not be at the hospital unless you seriously cock something up. I have a detailed understanding of these things, and I still build for series boxes at .4 ohm or so.
Long answer. In a series battery configuration, for all practical intent, the top battery in the stack carries the full current and voltage of the circuit. This means if you give the mod 2 batteries, its going to deliver up to 8.4v through that battery. If you then put a .2 ohm coil on it set to 8.4v, you are asking that top battery to deliver roughly 40 amps in nice round numbers. If you are using Sammy 30Qs, this is like double what Mooch says they can do safely. At best, you are dramatically shortening the battery's functional lifespan, at worst you are risking venting or thermal runaway. Thusly, my recommendation is to build so that you do not exceed the amp limit of the top battery at the desired voltage, in most situations, .4 ohm. I know, these are in many ways overly cautious recommendations. Pulse ratings, voltage drop, etc will play a part if you want to actually model or measure the current on the circuit. Whiterose strongly recommends the VTC5a in this mod, and I agree with him. I am limping by until my VTC5a order gets in from Illumn this week with some LG HD2 cells and some Sammy 25r5 cells that I had previously married as series batteries. In general I'd recommend you observe and adhere to the published or tested (by Mooch) discharge ratings of your batteries, believe in ohms law like the bible, and try your very best not to get on the news, because that hurts all of us.