Fire is certainly high up on the top ten list of mankinds most useful inventions. It allows for a more efficient utilisation of nutriants, is the basis of chemistry and the material sciences, allowed us to settle the Northern regions, was the main way of storing energy for most of our history and gave us an edge over other competing species. One can hardly contemplate the rise of a technological civilisation without fire, thus making the range of atmospheric oxygen content where fire is useful important for my worldbuilding.
I'm already aware of a number of limits for oxygen levels. If the oxygen content rises above 35% fires won't stop burning and runaway wildfires will keep the oxygen levels at or below this value additionally the human breathing range lies between 0.16 and 0.5 atm.
Especially with the first constraint in mind I'm interested in the range of oxygen content the atmospere of the homeworld of an aspiring technological civilisation can have while fire remains useful to them. Useful means that the use of fire gives them huge advantages in their primitive environment like heating, lighting and cooking without demanding extensive caring (read if one person needs to supply air regularly just to keep the grill going its too much) or being extremely dangerous (if lighting a fire is like opening a gate to a dimension filled with ill tempered fire demons and one spark will annihilate the camp in a fierce cataclysm it is too much).
What is the range of atmospheric oxygen content I'm looking for? Does partial pressure or fraction of the atmospere matter?
Use earthlike conditions as your answers baseline, only taking away or adding nitrogen to adjust for the varying oxygen content. Should changing composition or pressure lead to interesting effects feel free to mention them. Thanks in advance guys.