Hello
For me a good dbm is no higher than -65dbm...
For me -70 dbm is bad -80 is terrible. and so on. Anything above -65dbm is not good for me. At least i take that nubmer as a stand point when im doing site survey.
This number applies to 5ghz and 2.4ghz
You will notice that if you stand for example 150 feet away from an AP in an open space let say a 105 AP you will notice that you will get -60dbm on 2.4ghz and -73dmb on 5ghz and thats why 2.4ghz can travel farther than 5ghz.
The dbm values i just used in the example above are not accurate it something that i made up to try explaining you!
Hope that helps
Cheers
Carlos