Let's move the ball somewhat.
I am going to exclude the first two TC winners, because they raced on a different schedule. (Sir Barton's Belmont was 11 furlongs, and Gallant Fox's Preakness was before the Derby.)
Only three TC winners had a close call in the Preakness: Affirmed, Assault, and War Admiral. Affirmed and War Admiral had a single challenger who was a very good horse and they were way out in front of the rest of the field. Assault got a bad start, had to be used hard, and he and the runner up finished clear of the rest of the field.
I will use this video from ABC sports, which contains footage all the TC's except for American Pharoah, as the guide for this:
Omaha won the Preakness by 6 lengths over Firethorn.
War Admiral won the Preakness by a head over Pompoon, a very good horse. It was way back to the third horse.
Whirlaway made one of the greatest moves in the history of horse racing, moving in and out of horses and winning the Preakness by a distance over King Cole.
Count Fleet won the Preakness by 8 lengths over Blue Swords.
Assault got a bad start in the Preakness but won by a neck over Lord Boswell. They were well clear of the third place finisher.
Citation won the Preakness by 5 1/2 lengths.
Secretariat won the Preakness by 2 1/2 lengths over Sham, and it was a long way back to the third horse. Secretariat made a premature move in that race, and ran what was almost certainly the fastest Preakness up to that time (although exactly how fast he ran is forever shrouded in mystery).
Seattle Slew won the Preakness by a length and a half. His time, though, was the third fastest Preakness up to that time in history.
Affirmed won the Preakness by a neck over Alydar, a very good racehorse who also finished a head back in the Belmont. They were well clear of the third horse.
American Pharoah won the Preakness by seven lengths over Tale of Verve.
So if Justify wins the Belmont, he will have done so after winning the least impressive Preakness of any TC winner except perhaps Assault, in which he beat Bravazo, Tenfold, Good Magic, and Lone Sailor by pretty short margins.
I think the correct play is to be skeptical of TC attempts to begin with, but at the very least, I want to see a smashing victory in the Preakness, either in terms of winning margin, time, or horses beaten, before concluding a horse is going to win the TC. You simply don't win the TC three weeks after barely beating Bravazo while running a 97 Beyer.
