Choose "Classic f.lux" from the color selection menu in the f.lux window.
Set the color to 1200K. Or, you can also set it to 800K using the keyboard shortcuts, which you can enable if they're not enabled by going into the f.lux options and turning on "Use these hotkeys to control f.lux".
Change your location to somewhere in the world where the sun has already set or risen so that it's opposite you. So, if you're doing this at a time between sunrise and sunset, choose a location that's between sunset and sunrise.
Change the color temperature to 1200K (or, again, you can try 800K as well).
Now it no longer matters what your location is set to or what time of day it is because this sets f.lux to the exact same color temperature for both day and night, which means it will never change on you ever again - unless, of course, you change it again.