Guides
Manual mode, 1/1000s, aperture wide open, Auto ISO with the ceiling set to 6400, continuous AF, raw. Dial that in before you leave the house and you'll come home with sharp files of most animals you meet on a walk.
Elevation sets the calendar. Above roughly 9,500 feet the aspens generally turn mid to late September; mid-elevation valleys around 8,000–9,000 feet usually run from the last week of September into the first week or two of October; the foothills and the plains cottonwoods along the Front Range come last, mid to late October.
Two measurements settle most of it: the width of the furniture under the wall, and the height of the center of the art. Take 60–75% of the furniture width — an 84-inch sofa wants 50–63 inches of art, a 60-inch console wants 36–45 — and hang the center 57–60 inches off the floor.
The short answer is two-thirds the width of the sofa. The longer answer is that height matters more, and it's the part that's easier to get wrong.