I think I've heard clear up to 40ppm of nitrate is 'safe' if its truely FOWLR
...no corals or inverts.
I'd really aim for under 20ppm though, because it means your water is that much cleaner.
Are you running a protein skimmer? Skimmers will remove the stuff before it can become nitrate, as well as lowering phosphate generating compounds.
If all else fails, look into setting up a refugium, with some macro algae, like Chaetomorphia. it'll suck up some nitrate and phosphate.