Definitely not a case of too low Mg, but yes in terms of balanced to Ca, it is low, but I would rather allow Ca to drop to the 390 to 420 mark, as there is no benefit in keeping Ca above 400ppm.
Too low nutrients will hardly cause sps corals to turn brown, as it is usually a function of zooxanthellae population density increasing as a result of available nutrients that causes brown corals. Or, insufficient lighting, which does not seem to be the case here, assuming bulbs are in decent reflectors, even not, you should have sufficient light.
A No3 of 0.5 and PO4 is more than sufficient to brown out sps if combined with regular and excessive feeding.
Are you dosing a liquid carbon source such as Nopox?
Colorometric hobby grade test kits, whether they state "pro" or not for PO4 is not nearly as accurate as we need, so your PO reading is merely an indication that it is there. Rather bring that and your NO3 down to undetectable levels and feed your fish and corals.
Your tank dimentions are very similar to mine, in which i am running Flow at about 120 tank volume turnover rate, about 25 000 litres per hour.
When you say high flow, please elaborate.
You also did not mention your alkalinity?