So the Bortle rating is a rating of the current conditions with no consideration of one's experience or eyesight? Or does it rate different sites in optimum conditions, and therefore tell us something that is true of one site vs. another? In other words, under the same conditions Cherry Springs is darker than my home, which is true. Not sure I catch your drift.
If my eyesight is worse than John Bortle's, then I might rate a sky at 6 that he might rate at 4. What is the rating of that sky?
For you it is Bortle 6, for John Bortle it is Bortle 4.
Bortle ratings are not about optimal conditions, they are about the current conditions as seen by you the observer. This is part of the confusion websites like Lightpollutionmap.info have caused, they have hijacked the original system and tried to make it something it wasn't meant to be.
In the old days, we used colors to rate the different sites.. This appropriately inaccurate and made no pretense of being universal, my backyard is in a red zone meant it was in a city.. A blue zone was quite dark. A black zone is as dark as it gets.
The Light Pollution Atlas 2020 still uses the colors and does not try to associate the sky brightness with a Bortle rating. The author is a member of Cloudy Nights and contributes to discussions of light pollution..
The color scale: https://djlorenz.git...020/colors.html
While the Light Pollution Atlas 2020 is not a slick as Light Pollution Map.info, in my experience, it is more accurate. It agrees with what I measure.
Based on the fact that Researcher lives in Mexico City and can see only Zeta Herculis, a Bortle 9 seems likely. Since Researcher will be using 10x42 binoculars and can see Zeta Herculis naked eye and it is only 5 degrees to M13 and another 2.5 degrees to Eta Herculis, I think he will find it.