What would you consider to be safe levels of macros?
I haven't focused on this question in my experiments so far. But if adding excess nitrate results in crinkled new leaves, an induced calcium deficiency, then that would be too much.
Here's an old thread from 2008 about this phenomenon. JamesC experimented with urea vs. NO3 and the results suggest that urea does not inhibit calcium uptake like nitrate does. The entire thread is worth a read and you'll notice Barr dismissing urea as a nitrogen source. Ironically, Barr suggested to use urea about a month ago after dismissing it for years.
Dosing with Ammonia and Urea | Page 3 | UK Aquatic Plant Society
Just some extra information that may be important, in the crop literature, nitrate-N helps increase absorption of ammonium-N, something about the cation pushing the anion around. This increases growth than if only ammonium were applied. If this applies to submerged aquatic plants, then they should grow faster with a mixed source of N. I dont know if this applies to urea, however.
Also, excess potassium can inhibit calcium uptake. I've noticed this occurs quickly when K is more than Ca, but I'll need to experiment further to confirm. My tap is very soft, Ca ~10mg/L. If I add 12mg/L of K from K2SO4, some of the new Rotala leaves crinkle, but H. pinnatifida grows faster with the higher potassium concentrations.
Kekon has done these experiments and has observed the same phenomenon but at different nutrient ratios. Here's a thread more than 10 years old. He also asked how some can have high K and low Ca without problems. If that's true, then it has to do with different plants or maybe a third variable that prevents potassium-induced calcium blockage like high Mg.
50 ppm Ca and still Ca deficiency - Fertilizing - Aquatic Plant Central
Also, excess trace metal cations can also inhibit calcium uptake. This is why the crinkly new leaf symptoms sometimes occur when dosing high amounts of traces. This is what I've observed many times when I was dosing EI levels of CSM.
So to answer your question succinctly, there are many variables that affect nutrient uptake and inhibition. I don't think suggesting any amount can be done without also indicating other parameters.