I've been running LED's on my 7gallon SPS for a year and a half now, as an experiment. I didn't get color on the bottom of the corals before this setup. So with this tank, I went bare bottom, and crinkled aluminum foil under the tank. Worked fine. With this tank, I focused more on water quality, and stability, before really tweeking light. Anywho, I am running the AI Nano on this tank, and started with whites at 10%, and blues at 20%. My lights are on total of 8 hours, with whites running 4 hours, blue at 6 hours,mand royal blues at 8, each ramping up and down. After 6 months of dialing the lights in for intensity, all of the blues are at 100% and the whites are 35%. With the whites above 35%,mand blues maxed, I've found way better growth and color, especially underneath and between branches. I've dialed in the whites last, as growth was an issue, and found the corals explode with the intensities above 35%. The coloring in the tank looked a bit blah at 40%, so I backed it back down to 35%. Looks are subjective to one's personal preferences, I like blue tanks.
As for tank parameters, quality is relatively consistent. DKH is 8-8.4, CA 410-420ppm, Mag 1360-1400, PO4 .03-.08 (vodka), and Nitrate at .25-10. Color is best with params at DKH 8.1, CA 420, Mag 1380, PO4 .08, Nitrate 10. With the tank being so small, things are going to swing like crazy, and I'm happy to be keeping it within these params.
As for the use of a PAR meter, IME they are not the best tool to use on LED's as you won't ever get a true reading with each bulb being what it is, making consistency an issue. A spectrometer would be best, but you want to talk about expensive, considering a PAR meter isn't cheap.
As of right now, I would seriously go back to T5, as it's just easier to deal with, unfortunately, there aren't any T5 fixtures small enough. Next tank is an 18"cube, and a Hydra26...build in progress...