Light Pollution Map
The ATLAS 2015 overlay at lightpollutionmap.info most closely matches the trends recorded by SQM devices. The other overlays are not a good indicator of zenith darkness, but can be useful to pinpoint local light pollution sources.
The ATLAS 2015 overlay is based on the The new world atlas of artificial night sky brightness.
[Webmaster: I find that subtracting 0.25 from what the map tells me matches what I get from my SQM-L here in New Brunswick, when there is no snow cover and the transparency is average.]