Some interesting points there WWT. Good to get an ATCO's perspective. Aerocom as sated there are plenty of different methods to determine a safe distance. I was always taught (and still teach today) that for every 10,000 feet of CB, add 10 miles for off-track avoidance. E.G. 10,00 feet divert 10 miles off track, 20,000 feet 20 miles off track, 30,000 feet, 30 miles off track etc.
The biggest and most powerful thunderstoms I have witnessed anywhere in the world are right here in Oz. Particularly around the Kimberly region of W.A. If you draw a line from Syd across Oz to Perth, you will find some of the best CB activity anywhere. Whilst even the smallest CB should be avoided at all costs, the behemoths in the Northern half of Oz require the utmost respect.
Scouting around the outskirts of ANY CB with your wingtip in the cloud is an accident waiting to happen. Contrary to what has been previously stated, I was in a jet that rolled through 60 degrees and lost 400 feet in "clear air" whilst not giving a CB a wide enough birth. By radar we were more than 15 miles from the cell. I have never been that close since. Maybe an over reaction but I have never been caught out using the above technique.
Use whatever works mate, just don't get too close.