Yes, the 3.1 has a ball valve. The screw on the side is for air flow adjustment. I just find it easier and quicker to top fill, although on my 2s I do use the bottom screw... Huh, that's just screwy?
There are numerous online, android and probably apple calculators. And there's really two formulae to deal with, Ohm's Law (E=I*R) and Watt's Law (P=I*E), where E is volts, I is current (amps), R is resistance (ohms) and P is Power (watts). Simple to remember and use if you spent 45 years as an electrician
Now, as to your specific question, said 200 lb male of that stature can obviously
vape 5.26 watts. That's weight divided by height with the result divided by ohms. See? Simple