+1
That's what I usually do. You should also consider what your predicted record will be in the new league and how will that affect your attendance. Also consider that if you promote you'll get a bonus in attendance for the early part of the season, so use that to your advantage (wisely).
That was the exact purpose of this question, to find out what some of the wisdom on this would be. I know I'll want to boost the tickets for at least the first half of the season with the promotion excitement, and figuring out the right targets is pretty critical to success.
I feel like my arena is of sufficient size to bring in what I'll need to be competitive (12500), but I don't know where I'll be landing so it's going to take some studying once I promote. I scouted a handful of D-3 leagues at various points, and I see competitive teams with arenas of my size. Obviously some will be bigger, and I know I need to aim toward 20k if I want to promote again with emphasis on Lower Tier and Courtside seats first.