Well there is an experimental method for mechs that in the end you have to use somewhat anyway. Of course always test your coil before putting it on a mech. So you still need Ohm's and Watt's laws.
It is: If the coil runs too hot, add a wrap and try again. That will increase the resistance -- less power -- AND increase the size of the coil, both making it a bit cooler (less heat flux

). If the coil heats too slow, try reducing one wrap. If that makes the resistance too low, start again with thinner wire. Etc. It ends up being safe enough but how laborious depends on your starting point. Maybe it explains it better though.
A mech battery sees a certain resistance "out there" and coughs up power as it does in the face of that load, but it has no idea really what it's driving. You have to align the power "pulled" from the battery with the coil's actual requirements.
My method of finding the right power level for an atomizer is also experiment-based, and is best done without the complexities of a mech. First, decide your airflow and technique. Only then try coils. If a bigger coil doesn't result being able to set the power level much or any higher, you've found your personal power limit for that atty. An even smaller coil may work better. However a little too big of one is still better than one too small. If a bigger coil does increase optimum power setting, keep increasing. I didn't have to deal much with that at 15W since even smaller seemed improbable.