Hi and welcome to the board.
Regarding mechs, your head is pointing in the right direction for safety. To calculate maximum Amp draw on your batteries, you should use the full charge voltage of 4.2V as you state; not the nominal, half-charge 3.7V, as the former condition actually obtains with a fresh battery and yields the highest battery loading. Knowing only V and R, the formula for A is A=V/R; thus: 4.2V/.2Ω=21A, or somewhat in excess of the 30Q capacity and putting your safety margin in negative numbers. For batteries, I like to leave at least 10% on the top to keep from blowing my lips off, so the actual max permissible 20A battery loading is (MHO, of course) 18A. Backing that into the calculations-- I cheat, I use the
Ohm's Law Calculator, and
Steam Engine also has an Ohms page-- the lowest coil one can build is .2333(repeating)Ω. I really don't suggest that, frankly: For your first coil, or first dozen, I wouldn't go below about .4Ω, and for a couple of reasons.
Safety being one: give yourself some practice and breathing room.
More importantly, though, is because while your head is right about safety, coil building on a mech is a whole other world. A coil builder who learned how on regulated mods is missing some important lessons because of the ease with which the power can be adjusted on a regulated mod. On a regulated, if you want to vape a .25 coil at 30 watts, no problem. This will never happen on a mech.
Off to the right on the coil building page of Steam Engine is a "Heat Flux" box. Get used to looking at it because it's going to tell you how your coil will vape before you build it. The heat flux is simple: how much power is radiating from each square millimeter of coil surface. This one variable controls much of the flavor interaction in complex juices (you don't say anything about what kind of flavors you like) and, combined with the total coil surface area, controls cloud vs. juice consumption vs. battery life (duh).
Different juices react to different heat flux differently (lol). As you probably already know, if you start to lose the maple syrup highlight note in your patented, DIY Cinnamon-Apple-Honeydew-Grapefruit-Bacon-And-Eggs-And-Chicken-With-Waffles-Chocolate-Donut-And-Grits-Breakfast-Blend, you can get it back by backing off the watts by 2 or 3 points. With a mech, you have to build for it; there ain't no adjusting the watts once the coil is installed. So, one of the things you are going to find is two coils built with different wire
to the exact same resistance are going to have different heat flux, and thus vape differently on a mech.
As an example, Steam Engine informs me a .5Ω dual coil set on a 3mm mandrel using 26ga SS316 round wire must be wound 15/14 wraps, and will yield a heat flux of 70mW/mm² (milliwatts per square millimeter) at 30 watts (4.2V) and 60mW/mm² at 25 watts (3.7V); but the identical resistance wound in 28ga SS316 wire has to be built 10/9 wraps on the same mandrel, and will yield heat flux of 140 and 120 at high and low battery. Going down to 30ga wire, we see 7/6 wrap dual coils yielding heat flux of 280 and 240 at the same wattages you're actually going to get out of a one-battery tube mech with a .5Ω coil set before the vape gets weak from discharge. What that all means, in case this is confusing you, is the vape gets warmer and cloudier the smaller wire you use, all other things being equal. (Edit: note also the heat flux band gets wider with smaller wire; ie. more variance in the vape across the battery charge.)
Therefore, the right way to build coils on a mech is to match them to the juice you are vaping. Experiment a little: build different coils of the same resistance, but varying in wire size and mandrel diameter and find a combo that brings adequate cloud and eye-watering flavor across the entire wattage range you will actually be getting on your mech during the full charge--discharge battery cycle, and having done so,
note down the heat flux that brought it to you. Once you know what heat flux your juice likes between those power levels, you can then build "thought experiments" in Steam Engine yielding the same flux using any wild-azz combo of wire you like and have a pretty good idea how it will vape before you build it.
Cloud, my friend, is only partly about power. Mostly it's about how efficiently you can use that power and, on a mech, that's about coil building. I happen to like 18350 mods, but 350 batteries don't have even half the amp capacity 650s do. Still, I can get more cloud out of a .7 or .8Ω build on a 350 mech than the kids in the vape shop can manage with their 4ga, double-super-extra-hybrid-stapled-left-handed-monkey coils they paid 9 bucks each for from some coil builder because they don't understand heating that gigantic mass of wire to vape temperature with a single 18650 takes 4-10 seconds before they even start to get vapor off it. (The looks on their faces are pretty priceless, lol.) I tell them all the time: it's not about
how many watts you have, it's about
knowing what to do with the watts you have.
To answer the question you posed above, then: I like SS316 wire and Rayon wicks (I use them nearly exclusively), and I build mech coils between .4 and .5Ω for the 650 tubes and .7-.8Ω for the 350s, with heat flux between about 120-240 depending on juice and how sore my throat is on that day. I don't use high watts much; it's rare to see me top 40, and I am usually in the 25-30 range I used as an example. Whether or no this will suit your personal taste is subjective; only you can tell. I will say, though, if you find yourself building coils on a mech much lower than about .4, ditch the one battery tube, and buy a multi-battery box. It's a
lot safer. (Parallel box, I mean. If you get a series box, your coil resistance will have to double because your voltage has.)
Now, about that faux-hybrid top cap (shudder). Dude, I've been safely vaping mechs for years, and those things scare me peeless. It is so easy to get into trouble... but you already know this, and forewarned is forearmed. The only thing I want to say about it is battery rattle adjustment is absolutely critical. It is very easy to dent in the top contact of the battery from pressure on it. No matter how much your atty pin protrudes, once the top contact of the battery is dented, making contact with the atty brings the 510 negative threads closer to the outer, un-dented portion of the contact.
Inspect your battery at every change, and don't use it if the dent gets bad. You may want to reverse your battery in the tube, leaving the positive side down toward the button (if your mod will do that, not all will) as it is harder to dent the negative contact.
Finally, let me also thank you for your service. Be careful over there. Things appear to be heating up a tad in the East China Sea. I blog on politics and geopolitics a lot (in other venues and using a different handle, of course), having been a serious student of these matters for half a century now, and while I do not wish to inject that stuff into this board, I have to say I don't like the looks of things.
Vape strong and vape happy...
