I had a old guy tell me this and I actually kinda believe it
If the swirl is above the eyes, the horse will be hot.
If the swirl is level or below the eyes, the horse will be mellow.
Every 'hot', or more spooky/spunky/difficult has had a swirl high up, above the eyes. Loki and Red especially
My first mellow horse, Divo who takes everything in stride, has a low swirl. Roughly an inch or so below the eye line.