You can test the actual toxicicty of any Total Ammonia reading here:
http://www.hamzasreef.com/Contents/Calculators/FreeAmmonia.php
The directions for how to use this test can be found here:
https://www.fishforums.net/threads/rescuing-a-fish-in-cycle-gone-wild-part-il.433778/
When I wrote the above article I had to decide where to put the red line for NH3. Science has shown us that different species i=of fish have different tolerances for ammonia,nitrite and even nitrate. But, "There are currently
18,380 recognised freshwater fishes." Of course not even close to all these species might be jept in our tanks. But even if that number were a mere 1,000 sspecies, it is not possible for all of them to be tested to determine what level of ammonia, nitrite or nitrate will harm them or even kill them.
So, what I decided was the besy way to protect our fish was to put the red line at a low level, One tht wold protect everything. So I set that level at 0.05 ppm of NH3.
The other part of this is how resistant any fish might be to any given level of ammonia. By this I mean what lower levels of ammonia exposure might do permanent harm which we can not see. An example would be an ammonia level that damages the fills of a fish opermamently but which we can never see as long as the fish behaves "normally." But such a fish may have it's life span shortened by such damage. Stress is know to compromise the immune system of fish. And this will usually reduce it's ability to defend itself from ammonia.
When we fiest set up a tank the fish we add are stressed. They have been netted and bagged and who know what else. Then w put such fish into a tank which is new parameter water, new surroundings and who knows what tankmates. And then because we are cycling with fish present, we may have to do water changes. For new fish in a new tank this will be stressful. We know when fish are stressed or sick they tend to hide. What do most of out fish do during water changes? Mine tend to hide until the things in the tank get back to normal in this respect.
So, we have ti assume that ammonia and/or nitrite levels which cause ut to change water in a new tank will be stresful to the fish even as it lowers the level of these two things. And those things will also have stressed the fish as their level rises. And still we poften hoave no idea what damage might have been done to which fish during such incidents.
The above is why I do not support cycling with fish, ever. I know how to do a decent fish in cycle which is likely to protect the fish. But I will not do this, ever. At best, things will go well, at worst I might kill all the fish. There are a lot of bad things which can happen which do not result in death or which may not be obvious when there is damage but no outwards sign of it.
I smoked for over 45 years before I quit. I did pretty well for most of those years. Today I have COPD. I made the choice to smoke, my fish did not choose to have ammonia or nitrite in their water. I am human and I make mistakes. So, I try not to create situations in my tanks where my mistake can cause harm to the fish.
When fish are born in our tanks or we acquire tem some other way, they do not come with a label which tells us how long any one of they should live. And, we have no idea, if we expose them to non-lethal levels of ammonia or nitrite. of what we might have subtracted from whatever amount of time they might have lived had this been avoided.
Of course, ther above is just my opinion for what that is worth. Your thinking may well be different. As always, how your treat your fish is your decision.