and it has to be useful magnification, i.e. getting a reasonable image.
One night of very good seeing I got to 750x on the Tarantula nebula (surprisingly crisp resolution of stars at the core) and 625x on various planetaries and 47 Tuc using my 10" dob. I also used 500x and 714x to resolve the Sirius Pup.
Most nights however I can't go past around 300-350x, and on one night I couldn't even use 69x
