The ideal wattage depends first and foremost on the coil(s) you use. Other factors like the wicking, the airflow adjustment plus the strength of your draw and the design/type and dimensions of your atomizer (including your drip tip) also play a part, and, to a lesser extent, so do the juice type and the flavor profile of the juice, but after you add personal preferences on top of the sum of all these others factors, typically when determining your wattage you'll need trial and error more than almost anything else.