That's one way to do it in winter, if exactly 12 hour days suffice, and don't move too much. Manually adjusting the longitude every time I fly to the US would be a pain though.
What I'd really like is to tell it 'turn yourself off at sunrise, turn yourself on at sunset or 2100, whichever's later'.
Tell F.lux it you live on the equator in the settings. If it's triggering too early, move your fake location west.