Something I mentioned to Nick yesterday:
Tattoo machines are not a standard device. Different makers, parts, weights make all tattoo machine have their own "User manual" This means that what it works for one machine it may not work for another one ,even if they are from the same builder or model.
I tell people to run their machines to where the machine feels that is working right. There is not right or wrong voltage. Swearing by standards or by what works for this or that artists is the wrong way to make your machine perform right.
Learn to listen to your gear and feel it. Turn up or down the voltage "until your machine feels right"
A machine with more wraps will always require more voltage, More wraps stands for higher "ohms" in electronic terms.
"A unit of electrical resistance equal to that of a conductor in which a current of one ampere is produced by a potential of one volt across its terminals."
If people dear to understand that it would be easier to understand a machine.
Try, try and try, and don't follow trends, follow what works for your style of tattooing and machine.