I used to just eyeball bearing preload until a hub on a customer's $2,000 wheel set came back loose twice
For years I snugged cone nuts by feel, figured close enough was close enough, and honestly it worked fine on most cheap hubs. Then last spring a regular brought back a rear wheel I built for him because it had play in it after about 60 miles of riding. I pulled it apart, and the cones had backed off just enough to chew up the races, which meant a whole new hub and a rebuild I ate the cost on. Now I set preload with a dial indicator and check it again after a break-in ride, and I have not had a comeback since. Any of you still doing it by hand, or has everyone switched to measuring it somehow?