The mpv 20w wont fire below 1ohm and it has a maximum output of 6amps. If set up properly it will work ok, but to me 6amps is not sufficient power for an rda. Others may or may not agree..
So, any coil or combination of coils higher than 1ohm will allow the device to fire.
Once youve wrapped a coil, you can only decrease the ohms by taking off one wrap at a time. To increase ohms, you need more wraps on your coil, or a larger diameter.
The kanthal gauge you use, the diameter of your coil, and the number of wraps will determine how many ohms your coil is.
This site is your friend,
http://www.steam-engine.org/ you need to know how ohms and volts and watts and amps work.