Don't follow watts per gallon, that is garbage. Even the LED
panels that have 3w per diode isn't a good measure of the lighting being provided.
In general, you are looking for PAR and PUR. Par is much more common to be seen, with pur being a more accurate reading on what your coral will need. These meters are expensive.
When you see things talking about the kelvin of lighting, we are talking more about the spectrum of light. If you remember having light go through a prism the first time to make a rainbow, it is exactly that.
Start out with this. Stick as hard as possible to BRS for your knowledge here. There is a ton of misinformation out there that people still believe. When I went through their videos, even with an established reef going at that time I learned something.