On my former reef tank I actually measured my par using an apogee meter with still water and aggressive surface movement. At a depth of 24" I would measure approximately 350 uMol with still water but during aggressive surface aggitation it would fluctuate between 345-355. From a practical standpoint it averages out and wasn't something I was concerned with.
For reference this was with 108 leds and 16 x 39 watt t5s over a 265 gallon tank. These results may not scale well with less intensity.
Thank you! That is very good to know. And, it is about what I thought would happen.