It actually doesn't matter. It's whatever is most comfortable for you. There is no right and wrong answer here. It is all up to you and what feels best to you. Remember, most monitors don't have good color accuracy, so your settings in f.lux aren't the actual color temperature that's being emitted by your monitor. The numbers are really just for your information - like reference points.
So, you have to experiment. You should also experiment with your monitor's brightness setting because even 6500K at a low monitor brightness can be far more enjoyable than a bright monitor at a very warm color temperature because of the superior color accuracy.
I don't want to influence your final setting too much, but keep in mind that you don't want the monitor to be brighter than the ambient light and your surroundings. If your monitor is the brightest thing in your field of vision, then it can cause eyestrain and eye fatigue and possibly a headache.