0.03 is the number i see commonly quoted as a targeted level for PO4. Your test accuracy, as earlier mentioned, is about this, so anything from 0.00 to 0.08 would be within a range I would be happy with. If it was on the higher range in there and I were having algae issues I might try running more GFO, but I would say with your current levels changing it now would have little to no noticeable affect. Pulling PO4 lower than about 0.03ppn will likely result in bleaching of sps and slowed growth as PO4 acts as a seed crystal in the coral's calcification process. If you are running a system that is having algae issues with PO4 around the 0.03ppm, look at your sources and sinks for PO4... you can have low levels while still cycling a lot through the system, it just gets taken up quickly by your sinks. In terms of cyano or Dino's, I often see issues during summer with them in many tanks. I suspect it has to do with increased silica levels in the tap water as my RODI system seems to get plugged with brown scum much faster during summer months and tank blooms seem to be much more common during summer (from what I've seen). I have had less issues since running RODI, but that's just me
Dan