Tomlinson Distance Ratings are an interesting tool that has been added to the DRF's version of Past Performances.
In a nutshell the Tomlinson Rating is meant to quantify a horses supposed proficiency at a certain distance based on the results of their sire and damsire's progeny. The higher the rating the better a horse is supposed to be suited to the conditions.
Tomlinson Distance ratings are calculated for 4 different categories of distance so it's not as if sprinters have a target range of 200-275 and routers higher than that. Every time you see the ratings listed in the Past Performances they are based on the distance the horse is currently contesting. Every horse therefore has 4 different Tomlinson Distance ratings, one for each category. You can read a more detailed explanation by visiting this page at the Daily Racing Form.
After determining that what they're measuring is useful my main concern with any figure is always how to make the best use out of them. I have long subscribed to the theory that breeding is a terribly overrated angle because it's rare for a horse to be an exact reproduction of their sire or damsire. Most horses are much worse than their sires and the type of horses that win Classics are usually better than their sires. Once you have a sufficient sample of races it's generally easy to tell what a horse is best at. You don't need to do a detailed lineage analysis to find out what they're supposed to be best at. The lone exception to this modus operandi is when assessing horses that are doing something new for the first time. The Belmont is one of those occasions.
Much is made about the fact that the Derby is a sophomore's first chance to go 10f and we never really know how they'll like it. But the Derby is only one furlong longer than a distance that the entire field has prepped at, with few exceptions. The Belmont is a full quarter mile further than any race a contestant has run. You will almost never find a race where no entrant has ever run within a quarter mile of the intended distance, except for first time starters. That makes bloodline stamina projection more important than ever and I would add that it's important for first time starters as well.
So how can you use Tomlinson Ratings to good effect in the Belmont? I always prefer averages to arbitrary lines because a horse does not need to beat some line in the sand or even runners from previous years. They only need to beat the horses they're facing. So the question changes from "Do they have enough stamina to win a 12f race?" to "Do they have enough stamina to beat the rest of this field at 12f?"
My attempt at answering the latter question looks like this. Take the Tomlinson Distance Rating for every horse in the field and come up with the average. Regard any horse with a rating lower than the average as one with suspect stamina. Horses that have an asterisk beside their rating should probably be given a pass since the asterisk denotes insufficient sample size.
Horses that did not have a Tomlinson Distance Rating that was equal to or great than the field average have gone 32-1-1-4 in the Belmont since 2000. Commendable was the only outlier but horses like Bob And John, Giacomo, Funny Cide, Invisible Ink and War Emblem could have been avoided.