Regardless, there is a (vague) point at which the light provided by the sun starts to change in color temperature (which I assume to be at or near the time that the sun appears to start descending below the horizon), and another (vague) point at which the direct light of the sun becomes insignificant as compared to other sources of light. Flux's time of change should be comparable in orders of magnitude to the amount of time between those two instances.

The length of official twilight can be approximated at half an hour (though it varies greatly based on time of year and latitude). I'd be perfectly happy if Flux approximated even that. But it's literally like 20 seconds.