This is a minor point, but given the desire for realism I believe it is one that should be made.
Humans have a requirement for a minimum partial pressure of oxygen. No change needed there. There is a maximum partial pressure of oxygen above which oxygen is toxic. While one might quibble that the partial pressure ought to be higher than 0.3 (0.5 is more likely correct), the concept is valid.
I do not believe, however, that there is any requirement for the percentage of oxygen in the atmosphere. All biologic reactions are based on partial pressure. The human body does not know how many other molecules are flitting about, nor does it care. While I am not certain that physical reactions, such as combustion, are driven the same way, I suspect that is the case. Reaction rates and equilibrium states are driven by Gibbs Free Energy which, for a gas, is based on partial pressure. The presence or absence of other molecules has no effect. This is not the case for solutions where concentration is key, but the analogue to concentration for a gas is partial pressure rather than % composition.
There is a minimum requirement, however, for total pressure. This is due to the effects of low pressure on boiling points and other state changes. So you could get to a similar requirement based on total pressure. Rather than having a maximum oxygen percentage of 30%, you could have a minimum oxygen partial pressure of 0.1, a maximum oxygen partial pressure of 0.3 (or 0.5) and a minimum total pressure of 0.33.
