I just checked my DNA device with a VTC4 that just came off the (USB onboard) charger a couple hours ago, so it still has a near full charge.
With a 1.4 ohm coil at 7 watts, which should be 3.1v, it says it is firing at 3.9v and the resistance figure is flashing. At 3.9v, on that 1.4 ohm coil, it is actually firing at 10.9 watts.
With a 2.0 ohm coil (the highest I have here) at 7 watts, which should be 3.7v, it says it is firing at 3.9v and the resistance figure is flashing. At 3.9v, on that 2.0 ohm coil, it is actually firing at 7.6 watts - just above the minimum wattage setting. My guess is that if I were to let the battery discharge down to 3.7v and fire the 2.0 ohm coil at 7 watts, it would stop flashing. If I can remember not to plug it in and let it discharge that low, I'll try it again and report back.
So, if 7 watts really is the magic number you want to hit, and you want it to regulate all the way through the voltage range of your battery's charge, you will need a 2.4 ohm (or higher) coil. At 4.1v, that 2.4 ohm coil would fire at 7 watts. As the battery discharges down to 4.0v, it would start regulating the charge up, and would continue doing so throughout the discharge of the battery.
My recommendation is to experiment a little with higher wattage, though. That 1.5 ohm coil in your kayfun is right around what I've been told was the sweet spot for that tank and this chip. The minimum wattage that would allow the DNA to regulate throughout the charge of the battery is 11.3 watts. If you set your wattage to that or a little higher, it should continue to regulate all the way through the charge of the battery.
I would actually give 14-15 watts a try. I run my Fogger 4.4 (which is the only RBA like a kayfun that I own) with 1.4 ohm coil at 16-20 watts depending on the juice I have in it. In fact, I find that I end up running all of my atomizers/tanks (which range from 1.3 - 2.0 ohms) at wattages that come out to 4.5 - 5.5v, depending on the juice and the type of coil/wick I have in it. It gives a quick to build, slightly warm vapor that is by no means hot/burnt or extremely taxing on the device.
Don't be afraid of a little more power. At 1.5 ohms and 18 watts, you're smack in the middle of the range of what the device was designed to do- a good, safe place to be.