Now, let's talk temperature: what to set it at?
Let's kill one myth right away: dry cotton chars at 420F, yes. But even the dryest cotton wick that has juice left in it it is wet enough to withstand a lot more than that. Forget about the burning temperature of cotton, it doesn't matter one bit.
PG and VG don't vaporize at the same temperature. PG turns into a gas at around 380F, and doesn't break down into nasty chemicals when you overheat it. VG vaporizes at around 350F, but more importantly, breaks down and turns into harmful acrolein at around 520F. As for flavorings, well, who knows... VG also needs a higher temperature to generate the same amount of vapor density as PG. PG also tends to "snap, crackle and pop" in the atomizer, while VG doesn't.
All juices are a mix of PG and VG (unless you vape pure PG or pure VG). So you'll want to dial a temperature between 400 and 500F depending on your particular juice. Why these values? because the DNA40 has a +-10F temperature measurement tolerance, to which I personally add another +-10F for safety - so +-20F.
For what it's worth, I personally dial 410F for pure PG juice, and and 480F for pure VG juice. But it's not really critical: 410F for pure PG juice gives me just the right amount of "snap, crackle and pop", and 480F gives me a thick, dense, flavorful vape with pure VG.
On top of that, the state of the coil also plays a role in its ability to transfer heat to the juice: a clean coil readily heats up the juice, while a dirty coil tends to overheat and transfer less heat to the juice. The gunk on the wire acts as a heat insulator. So if your coil is very dirty, you'll want to jack up the temperature to get the same density of vapor - without going over 500F, for obvious safety reasons. If you can't get a satisfying vape at 500F, it's high time to dry-burn the coil and re-wick it. But you'll do it well before that happens, believe me: the taste and vapor production will be quite underwhelming

Set the temperature to whatever gives you the best flavor/vapor density for a given power level, provided you don't go over 500F. If you stay under 500F, I can guarantee you your cotton won't burn and will stay nice and clean for a long time - unless the juice itself starts coloring it.