The problem here is that the
Bortle scale has nothing to do with the colored maps, phone apps, etc. Here is the original article from Sky & Telescope back in 2001 where John released his scale.
https://skyandtelescope.org/wp-content/ ... yScale.pdf
If you look at the criteria in the article you should be able to accurately determine your
Bortle level, at least at the time you are observing and as you see it. This can change because it is meant as an in the moment assessment with the naked eye. As you will note, John includes an NELM for each level. So you are on the right track there, However, those estimates should be made at the zenith and by employing averted vision to gain the most accurate perspective. The main problem with NELM estimates is that they are very personal. You could line up ten people side by side at the same location and ask them to do an NELM estimate. While you may not come up with ten different estimates, they certainly would not agree because of personal optical differences and visual observing experience. So your estimation of the NELM there applied to the
Bortle scale has a personal sense to it. That then leads us to the most often cited criticism of the
Bortle scale - individuality. Because we are optically different and of varying experience levels and abilities, how we interpret our sky based on the
Bortle scale may show some variation as well. I think John tried to get around that by using multiple criteria under each category.
Further, while the maps, apps and
SQM meters have their place, they look at things a little differently than does the
Bortle scale. As I mentioned above, John devised the scale as an in the moment naked eye visual method to assess one's sky based on a set of criteria. It is a sliding scale that can change from night to night or even hour to hour as conditions change. I have seen this particularly at our dark site house, but also under our suburban sky at home. Our sky at home used to be typically a
Bortle 5, but over the years it has moved closer to a
Bortle 6 unfortunately. A fate many folks are experiencing as suburban/urban sprawl continue.
The
LP maps are based on satellite imagery with an algorithm applied to estimate the spread of light glow over terrain. By their nature they are static with somewhat defined borders. Of course
LP does not adhere to borders, and the maps do not take into account small localized aspects that may make a specific site less usable than what one may expect based on the maps alone.
The apps and
SQM calculate the sky quality based on its ability to detect sky glow. While these readings can and do vary night to night and even hour to hour, they still are not a reliable fit to the
Bortle scale because they are using a different methodology.
All three ways are attempting to do the same thing, to help you determine the quality of your skies. But they do so in different ways and any correlation between them is purely coincidental and not wholly reliable. That doesn't make one method unreliable, simply that they aren't intuitively linked.
As an aside, I can tell you that John particularly dislikes all the attempts to draw a direct link from his scale to maps or devices. Those attempts were done by others, not him. As you can see in the article there are no "colors" associated with the different zones because that thinking was not part of the equation, and the
SQM and apps were not around yet. All attempts to correlate any of those things to the
Bortle scale were done by others in an attempt to link the scale to their methodology, and presumably make things easier.
I've had exchanges with John and I can tell you he bristles at any comparison between the methods. He is quite protective of his scale and still believes it is the best method to employ, and mostly eschews the maps and devices. I utilize the scale myself, but I also utilize an
SQM-L from time to time. I don't wholly rely on it, rather I rely on my own eyes most of the time. I find they are the best method.
I have employed the maps from time to time to try to find locations that might be suitable, but of course each site must be vetted. I have found seemingly useful locations that had very localized issues such as pole lights, or vehicle traffic driving by frequently or even very small bodies of water that increase fog conditions under certain circumstances. So the maps do not tell the whole story. Nor do the apps, and some would argue nor does the
Bortle scale.
So I believe you are on the right track here by using your eyes to gain a better understanding of your localized conditions. In the end they are the most reliable tool you have.