Start with Ohm's Law. Voltage= Current x Resistance. There is also Watt's Law, Power (watts)= Current x Voltage.
Obviously PVs deal with watts and voltage so remember these two formulas Power=Voltage^2 / Resistance if you want to know how many watts you are using and Voltage= square root (power x resistance). The second is nice if you have a VV and want a specific wattage and you need to know voltage.
If not of that made much sense then hopefully this does: Most people develop a sweet spot based on power. More power means a warmer coil that produces more vapor but will consume liquid faster and faster drain on battery. Example: I prefer about 12 watts because it is warm, I get good vapor production but it's not too much to where I'm coughing till I pass out. Your typical eGo with a medium resistance (2.5 ohms) will range from about 6.7 watts to 4.3 watts depending on the charge at that time. The mech will act in the same way but without the electronic protections obviously. Higher power also should have sufficient wicking in tank so that you do not get a dry hit, hence why people move on to rebuildables because it can be tweaked to user preference.
I'm not into mechs, but from what Ive read you need to make sure you have a safe battery for what you want to do. You have no safeties/protections aside from you letting go of the fire button and common sense on a mech.