It's not clear why this is necessary. If f.lux is monitoring the file for changes, it would be preferable for it to establish some kind of callback with the system to be notified of changes rather than open/read/close the file every second.

On my system, the file is located at: C:\Users<username>\AppData\Local\FluxSoftware\Flux\media\preset.json.

See: https://stackoverflow.com/questions/931093/how-do-i-make-my-program-watch-for-file-modification-in-c