I realise this is slightly "how long is a piece of string" and that there is no "benchmark" in absolute terms for dBm measures but I see that in various places in NZ I can get, via my Telecom Sierra T stick, measures ranging from -65 (about the best it seems) to -95 (towards the bad end).
The point of knowing is that if / when a website strats doing odd things, amongst the possible causes (the site itself, the browser settings etc) one may be signal strength.
All wisdom much appreciated.