Watts/gallon was a rule of thumb back in the power compact fluorescent and MH days. It worked reasonably well with early t5 fluorescents as well. But newer t5 and all leds really don't work with that rule.
PAR (Photosynthetic Active Radiation) can be measured reasonably well with a fairly expensive PAR meter ( Google Apogee PAR Meters). PUR (Photosynthetic Usable Radiation) is different for every species that does photosynthesis and is next to impossible to know or measure. I'm not sure why people continue to even talk about it.
A good PAR at the bottom of the tank is between 150 and 250. Zoas and soft corals at the lower end and lps and sps at the higher end, however most zoas and softies can tolerate the higher PAR very well. The sps don't do well at all with low PAR.
Most 3 watt/led fixtures will provide way more than the PAR you want, even cheap Chinese fixtures. I'd just avoid the Current, Marineland and other fixtures that use 1 watt and even weaker leds unless you are lighting a very shallow tank, like 8" to 16" max.
