I have always been curious what other locals rate their skies at. What do you consider poor or average seeing for the Central Valley?
Whether it's my actual seeing ratings that are off, or if I do have better/poorer skies from what others believe makes no difference to me as long as I'm satisfied with the end results. IMO, I'm much more affected by poor transparency.
My backyard seeing conditions haven't changed much over the last 25 years using the same APO, & same EP magnifications I used in the early 2000's. When I compare observing from skies others astronomers consider premium (seeing wise), my location produces good results IMO. When I log seeing values as mediocre, I usually word it like "I'm having short periods of very stable seeing conditions." I also consider good as better than mediocre.
Perhaps it's lower expectations on my part? I do find my 4" refractor isn't generally able to be pushed past 240x on the larger planets and double stars, so would that be considered poor seeing conditions? I'm usually more than satisfied running my NP101 at 154x and on a good night of seeing, maybe up to 215x.
In most cases, how unstable the stars look visually, gives me an idea the seeing might be substandard, average or possibly above average. The OP is asking how to rate the sky w/o a scope or binoculars, so the twinkling stars method works but doesn't determine whether I set up or not.
Without respect to aperture (since I use a range of apertures suited to where I find the optimum during a session):
Backyard seeing here on the valley floor is often ~150x level for planetary, that is poor seeing, and 100x level seeing is not uncommon. A night that supports about 225x decently is mediocre and that is fairly common. I prefer something in the 275x range to start getting the sort of planetary detail I am after, but that is rarely well supported here. Nights that will allow me 350x or so are uncommon--and have become far less so in recent years--corresponding with replacement of orchards to the S and W with new subdivisions. It has been years since I had 400x+ planetary night in the valley. The 350x+ nights are where things start to get really interesting, especially when I can bump 400+.
Where one finds the optimum for planetary with a given scope in excellent seeing is a personal thing, so we each have our own factor. However, when seeing is a big limitation I have noticed that folks tend to run about the same level. Club nights when I'm finding 150x about all I can stand with the 8, 10, or 20", I discover that everyone else is stuck around the same magnification in their 8" and larger scopes. On better nights folks will be pushing more.
On nights of good/very good seeing I tend to find optimal for planetary/lunar is around: 120x with the AT60ED (sometimes 144x for a bit more scale, but not detail), 144x w/AT72EDII, 150-171x w/Orion ED80, 193 to 220x for the Orion 110ED (FPL 51 type doublet, shows some color). I have more history with the 8" SCT in very good/excellent seeing and have always found it topping out for planetary at 290x nominal/310x+ actual since a 2" TV w/high hat adapter is used--but it almost never reaches that level out here, which is why I had to augment my normal planetary set of 9 and 7 Naglers with an 11 and then a 13 to get me through typical evenings. My son's 10" has supported 357x well when the seeing was very good, and 417x briefly when the seeing was at its best before declining again. The 20" has been very limited locally, 357 and 417x for planetary on the best nights; while in East Texas 500x was not uncommon and I sketched Mars at 714x with it. Back then I was planning to try albedo mapping Ganymede...life got in the way, and it hasn't been feasible here due to seeing.
I get better average seeing between 4,000 and 8,500 feet, but that still doesn't compare with what I was accustomed to for my first decade of observing. The poor seeing in the valley, along with the rapid increase in light pollution have served to "extinct" my backyard observing enthusiasm. If the seeing was as good or nearly as good in the back yard as it is at my dark sites, then I would do far more backyard observing than I do, particularly when the Moon is up. For some folks it is the other way around, their backyards have better seeing (but are bright), so for them it is merely a matter of switching target types.
Very tight double stars are different with respect to optimum magnification, because the contrast is greater, so if the diffraction pattern is discernible I can push substantially higher to see more, rather than beginning to lose modest contrast planetary detail. Scale is generally an ally for close doubles. The spurious disks can punch through the mess as long as the seeing is about 3/10 or 4/10 for the aperture. However it takes better seeing to identify close high delta magnitude companions at higher mags.
6/10 seeing with a 4" is about 4/10 with an 8", and about 2/10 with a 20", and about 7/10 with an 80mm. For reference, 6/10 with a 6" aperture is listed as "fair to good" on the Pickering scale, while 7/10 is "good". Of course, 6/10 with a 20" is about 9/10 with a 6" and rated as "excellent."