The thing you need to realize is that wattage is completely personal taste, and that preference can even be different for different juices. Right this moment I'm vaping on a 2.3 ohm coil at 4.4 volts, which equates to 8.417 watts. For me, this is the bottom end of conservative, and if I wasn't trying to get rid of some sub par juice that tastes real bad cranked up, I'd be running it hotter, since I usually like between 10.5 and 12 watts on a vape.
A good rule of thumb starting out is to take the ohm's of your attachment, add two to it, and set your volts to that number. Then go up or down as needed from there. When you get it really dialed in exactly how you like it, you can then use a calculator, like this one:
Ohm's Law / Watt's Law Calculator for example, to find out what wattage you are liking(just feed in any two numbers and it will fill in the other two). Then you could use that same or a similar calculator to estimate your settings for other attachments.
Here's an example: Lets say you screw on a cartomizer and you know that cartomizer is 1.8 ohms(because you tested it, not because it says that on the side of it) So you have this great new flavor that you tried a sample of at the vape shop, and you've loaded this carto with it. So you set your lava tube to 3.8 volts, and take a drag. You don't like it much, it's too hot and you can't even taste the subtle hints of that fantastic *whatever* flavor that you tasted at the testing station, so you crank it down, .1 volt at a time, until you figure out hey, this Super Glittery Unicorn Mellon Ball juice tastes just perfect for me at 3.5 volts. So you have a few other cartomizers you want to fill with it because you need backups, and you test the ohms on them and they're 2.1 ohm and 2.2 ohm(notice all of these say they're 2.0 ohms on the side of them, labels lie) but you NEED to know what voltage you'll set these ones to. Sure you could do the same hunting for the right voltage again, but for arguments sake lets say you went and used that wattage calculator, and fed in your 3.5 volts and your 1.8 ohms to discover that you love that flourescent pink liquid at exactly 6.80556 watts.
Well, now you know, that if you are using that juice, you can use that same wattage calculation, plug in that wattage and the resistance of your attachment, and it will tell you exactly what to set your device on for your perfect vape, every time, 3.8 and 3.9 volts in this case. If you dig out a 3 ohm coil? 4.5 volts. A low resistance 1.2 ohm coil? 2.9 volts. You can calculate it for any attachment you know the ohms of with any juice that you know your preferred wattage for.
Or you can spend the money on an actual variable wattage device, set it to 6.8 watts, and never have to worry about it no matter what attachment you screw on, because it will pick the correct voltage to make your ball juice boil perfectly, every time.
Anyway, that's long winded, but maybe it helped someone.
