Maybe this will help answer the transparency question :-
http://unihedron.com/projects/darksky/faqsqm.php
"The SQM's readings are assuming 'best transparency'.
You can get an updated definition of the transparency in your area from:
Attila Danko's
Clear Sky Clock for North America.
Andrew Cool's
SkippySky Astro-Weather Forecast for Europe, Australia, and North America.
Also, frequently local weather stations can provide "visibility" and "relative humidity" numbers which could potentially be used as surrogates for actual transparency measurements (which aren't possible with a handheld meter)."
Anyhow my "best" SQM was with some of the Takahashi Star Party
meet in Yass back in 2016. I didn't measure it but someone did and quoted 21.67.
But around Japan, in a gondola carpark in Tateshina, I had 21.30, whist my usual astro spot in Chiba is around 20.97 at best. Whereas central Tokyo is a joke 15.xx or 16.xx.
In my family home in suburban Perth, I had 18.95. Can't remember what I had for Canning Dam, W.A. though.