I personally run at 0.03 ppm of phosphates, but I also feed 5x a day and very generously. So my system gets daily additions of phosphates but I aim to pull it out just as quickly. I would probably recommend most keep at 0.08 ppm and go from there. If your corals still don't look healthy and with good color even with 5-10ppm of nitrates in the water, maybe let your phosphates go higher. Me personally, have always had RTN issues above 0.18 ppm phosphates but that just may be my tanks.