I say acceptable as it will cause no long term harm.
The only research I have found on the effects of nitrates on fish is Nitrate toxicity to five species of marine fish by Pierce, RH; Weeks, JM; and Prappas, JM reported in Journal of the World Aquaculture Society. Vol. 24, no. 1, pp. 105-107. 1993 in which it is pointed out that earlier tests have shown that prolonged exposure to nitrates over 100 ppm may be detrimental to fish.
Now as far as I understand, marine fish are more succeptible to higher nitrates due to their method of osmo-regulation. There may be other factors that increase in a tank which are not tested for, but I am not aware of them.
If you have any peer reviewed scientific papers to prove the above wrong, I will gladly hear it, but until then I will continue to say that 100ppm is the level at which you need to worry about doing things. I am currently hoping to find some more on the effects of nitrates on fish, nut again, I can only quote the science I am aware of.
A number of ray keepers get jittery over levels. Talk to CFC who has them and has noticed they aren't as fragile as a number of sites would have you believe.
It's like SW, everyone says 20ppm is tha absolute max when there is no scientific evidence at all to suggest that and a number of fish live perfectly happy at lfs with nitrates of 80ppm.