Ok, I know nothing about the mod or the tank but you did a fine job using the wire calculator to end up with .3Ω. That resistance is perfectly safe in anything I vape on. The leg length is the length of the legs from the coil to the hold down post. Usually less than 5mm but you will see if you change the leg length between 1mm and 5mm the difference in resistance in negligible. Not to worry. Set your mod power for the vape you like. Start low, maybe 30 watts. The vape will probably be week and lacking flavor. Turn up the power a couple of watts and try again. You will find a sweet spot you like best. When the power gets too high you will start loosing flavor and eventually get a harsh vape. Using a .3Ω build you will have a large range of power that will produce reasonable vapor.