Dual monitors - now working!
A Dell laptop rebuild (Win 7) resulted in f.lux only working in the main screen (the one with the taskbar, and actually a sep screen via HDMI). The laptop screen was its normal brightness. It suddeny worked on both screens when I UN-ticked "Use display data for better colour accuracy".
Hope this helps someone - perhaps this is also a workaround to the 'apply f.lux to one monitor only' feature request...