There just isn't an easier way right now. PAR and PUR are it in the context of planted tanks. Not all LEDs (or even fluorescents) are created equal. There's variations in spectrum (as shown in spectral graphs, not just what us humans see as visible light...that's just a facade) and bandwidths, wavelengths, etc. Lumens or watts simply can't translate into effectiveness. You can easily be bright (lumens) but be supplying your plants with inadequate full-spectrum.
LEDs emit light in very narrow and specific bandwidths. Unlike, fluorescents, like t5 Geissemann (sp?) midday bulbs for instance, that have a broad spectral output... doesn't compare to a cheap t5 bulb bought at a hardware store.
The taller a tank is, the more light has to travel through. If you ever been SCUBA diving, the deeper you go colors become drab and all look blue. Red light gets filtered out first, blue wavelengths penetrate deeper.. But in a planted tank, you really need full spectrum (especially the red ~660nm supplemented). Take a look at BuildMyLED.com. They are really doing LEDs right in terms of full spectrum LED for planted tanks by supplementing specific bandwidths (reds, green, blue, etc... measured in nm) with enough power and optics to penetrate deeper tanks.
If you got an all white LED fixture that's rated 6500k, most likely it will be very blue heavy on a spectral graph (FYI: white light emitting diodes are actually blue chips with a yellow phosphor on it). Plants can use the blue (~460nm) but it's like starving them of a well balanced meal. They'll grow, but not optimally.
I tried this once with two Finnex Ray 2 fixtures on a 26g that was 21" tall. I pumped in CO2 and ferts. I was getting well over 80 to 90 PAR at substrate. Plants grew nuts, so did algae... and to my dismay, my high light plants that were suppose to develop red leaves (i.e. rotala colorata) in high light faded to green.
