diabetes.co.uk is much more than a forum.
It is also an amazing library of diabetes information.
For example, if you go to the Home page, type "BG converter" into the search box on the top right, the first item listed is this: https://www.diabetes.co.uk/blood-sugar-converter.html
You may also find the pages about Hb1AC useful. This uses different units (it measures something different) but you can get an estimated average BG from Hb1AC. If you search for "Hb1AC" you will see all the information.,
I thoroughly recommend taking a look around the website starting at the Home page and following the links and menus.
I think it is a hugely overlooked resource for information.