I think it would be great to have a slider for Nitrous Oxide which controls what throttle percentage activates the injection.
But it's reasonable! Nitrous increase the engine power, if you don't press 100% throttle, it means you don't want to use the whole power of the engine, then you don't need extra power from Nitrous
Where is it ? Yeah, but on a drag strip or example, when i don't want to press full throttle at the start, when i am able to press the throttle 100% it makes a very sudden power boost.