As far as the resistance increase for Ni200 goes, say your coil cold is 0.1 ohms, and at 400F, it is at 0.2 ohms (just pulling numbers out of the air). If set to 30 watts, when it is at 0.1 ohms, the dna will be feeding roughly 1.7 volts into the coil. Once the coil gets up near 400F, and the coil is at 0.2 ohms, the dna will be still providing 30 watts, but now it will be feeding 2.4 volts into the coil. It needs to increase the voltage as the resistance increases in order to maintain the 30 watts. Since the chip is reasonably efficient, in both states the draw from the battery will be about the same (around 8 amps from a fully charged battery, more if the battery voltage is lower than full charge).
Once the wire actually hits your temp limit, say 410F, then the voltage/wattage will drop until the dna detects that the wire is below 410F, and then it will go up again, and then down again in a feedback loop, pushing a bit more or a bit less power into the coil to hold the 410F temp. The amount of wattage used in this state will depend entirely on how much heat is being pulled away from the coil. So if the wick is wet and you are drawing on the atty, then the wattage may stay near 30. If on the other hand, you have stopped inhaling, or the wick is dry, less power will be needed to hold temp, so it could drop all the way to 0 at times.
This is similar to the way your home thermostat works. If the house heat is off, and the house is at 50F, and you have your thermostat set to 72F, then when you turn the heat on, the heater will come on full, and it will stay that way until the house hits 72F. Then the heater will turn off, until the house cools to below 72F, then it will come back on, and off, and on, to maintain the temp. So during the initial heat up, the heater will be on full (40 watts), and then once it hits the target temp, it will turn on and off repeatedly, for an average power that will be less than full (something less than 40 watts). The DNA is similar, but it will smoothly reduce the wattage once it hits temp protection, rather than turning off entirely, since that could cause a pulsing sensation when you vape.
The best way to think about this would probably be to ignore anything you might have learned about ohm's law or other conventional wisdom for mech/vv setups. Wattage is everything, and will be the direct determination of how much power your battery is providing, up to the temp limit. Once the temp limit is hit, then the wattage will be lower. In an ideal setup, when vaping, the temp limit shouldn't be hit much, so again, back to just needing to consider wattage.
So consider the DNA to be a bit of a black box, if set to 30 watts, it will draw approximately 33 watts from the battery, and it will provide 30 watts to the atty, regardless of how the resistance changes. Then when it hits temp limit, the wattage will drop as it needs to. You could be using a 0.1 ohm atty, or a 0.5 ohm atty, or anything else, and it will be still using 30 watts (33 at the battery due to internal losses in the chip) right up until temperature protection kicks in (if it does at all with your build).
Regarding high ohm ni200 builds, you would be hard pressed to get a build up to 0.8 ohms. I've never built anything above 0.2 ohms, and those coils can barely fit in an atty. But assuming you could, or you were using some crazy thin wire, then according to the DNA 40 specs, it should still work correctly. But really you won't ever get this high. A 0.8 ohm coil, if wrapped around a 3mm bit will require 38 wraps of 30 gauge wire. 24 wraps of 32 gauge. The only one you might be able to fit would be 16 wraps of 34 gauge, but it will be nearly impossible to wick that coil without destroying it since 34 gauge ni200 is incredibly soft. The only time I use 34 gauge wire is when making a twisted wire, usually using 3 or 4 strands. And even then, it can be a bit hard to wick without distorting the coil.
Finally, regarding the limits that Evolv has published about the DNA 40, some of them are soft limits. You can build an ni200 coil lower than 0.1 ohms, or probably higher than 1 ohm, and it will still work. The chip just may not be able to provide the full 40 watts in those instances. This seems to be true for most of the clone dna 40 chips as well, but it might not for some of them.
So to make a long story short, whatever wattage you have set on the DNA 40, that will determine how long your battery will last. The ohms of the atty do not factor into this. So if you get ~10 hours of usage at 20 watts, you will probably get ~5 hours of usage at 40 watts, or ~20 hours of usage at 10 watts.
In practice, I get a full day's usage out of a 2500 mah battery with mine set to 30 watts. So I feel fairly comfortable going out for the day with only the 1 battery. Once you get your device, you will get a feel for how long your battery lasts with your usage pattern. You may get more or less than I do, but either way, picking up some extra batteries and an external charger would be a good idea. I've found that the Efest purple 2500 mah high drain batteries work well in the DNA 40, and you can find them for 6 or 7 dollars on most sites. A decent 2 battery charger like the nitecore will only set you back 15 or so, which means for 30 dollars you can have a few extra batteries and a charger and not have to worry about running out of power.
