agreed - everything we do is an estimation. More of an art than a science. PAR in Air gives us a good comparable baseline.
That said, I now have I week with the Ray II on my tank. While much less light than my HQI Metal Halide - I am getting much better growth with the Ray II than I expected (abet I expected to see plants gradually melting away - so any growth is much better than I expected). Lots of new shoots and leaves. Makes me more optimistic.
58 gallon oceanic, 250w 10k pendent, pressurized co2, eheim pimp #179 - 2217 and diffuser