Are you making your own
coils? Or just trying to find the correct resistance for your atomizers?
Either way, if you're trying to calculate correct wattage for your battery (voltage squared divided by resistance), you should always calculate for the stated voltage of your battery--in your case, 3.7v. The 4.2 (max voltage) doesn't last very long--most of your
vaping will be done at 3.7v and then slightly below, until the cutoff point (usually 3.2v or so).