I agree with Bright Giant. I am fairly new to astrophotography and have also been looking into this. I see from various maps that the same Bortle zone can cover a large range of radiant light indices. As the Bortle scale seems to be defined by light at the zenith I am assuming that a low radiance value is important because this I presume is affecting viewing at lower angles, say 45 degrees , which will be important when viewing ,say, the Milky Way. I am thinking you would want a lowish Bortle Zone, say 3, but in the UK 4 is more realistically accessible and a low radiance value. The map I am looking at www.lightpollutionmap.info quotes these and whilst I don't fully understand the units it would appear that 0.18-0.3 is very dark, 0.4-1.0 is achievable in more accessible rural areas. I hope this helps but if I am talking rubbish feel free to tell me because it would help me as well !