I can get by with 3.7v or 7watts on most devices. It depends on the coil design. I usually run a dual coil 1.7ohm a little higher than i run a 1.7ohm single coil. iCear30s seem to like a bit more power too because of the dual coil design. Im not sure about the ohms on the first iClear30 i bought but i ran it at 4v-4.2v on my original MVP.
I typically run either 3.7v or around 7watts on anything upto 2.2ohms and a single coil. After that they seem to need more volts and heat. Some of it depends on my mood too. Using a smok DCT tank with a 1.5ohm dual coil, the vape is pretty good at only 7watts or 3.8v but if i want a harder throat hit, i go upto around 8.5-9watts.
BTW which ever you set last on the MVP V2 is the setting it runs on. So if you change volts, its running on volts and if you change watts its then using watts as the constant along with ohms for the RMS equation. It appears to be the same for setting it on volts. Volts and ohms are then the constants and watts becomes the variable in the equation.
Setting it to VV takes Root Mean Square (RMS) out of the picture. Only when you put the device into VW mode does it do the math and determine for you the correct voltage to apply, based on the resistance of your coil, to give you the same power for any coil/head/tank you throw on your VW device. When you use your device in VV mode, it doesn't matter if you put a 1.8 ohm coil or 2.5 ohm coil on it. It will apply the same amount of voltage and you will get varying amounts of power based in the resistance of your coil.
5 volts on a 1.8 ohm coil = 13.89
5 volts on a 2.5 ohm coil = 10.00