Well it was a trick question: the coat is neither the correct length, too long or too short. The coat is clearly neither too short nor too long. So we might assume that it is the "correct" length but I am not sure the concept of "correct' length does us much good at all. The "correct" length measured in centimeters on one style of coat might be "incorrect" for another style.
What we are working to establish is "the length that for a given style of coat renders the image most harmonious and balanced", what we might call the "balanced length." The two button SB worn by Windsor in the photo is a perfect example of "balanced length."
Many writers on dressing have published rules about measures from the thumb, half the distance from this to that, the measure that covers the buttocks etc. that serve little use except as a very rough starting point. The proof of this can be seen in the above photo where a "correct" but "imbalanced" length of coat can be seen in the picture as well. Look carefully and study the dress of the man standing in the background to the right of Windsor. His coat arrives "correctly" at the knuckle of the thumb and yet the coat looks quite a bit too long on him. It looks like an overcoat it is so long.
If we compare the figures of Windsor and the man next to him we see striking differences in the styling of their dress. Our Monsieur Smoothjazz has been insightful in his exposition of two key components of Windsor's dress: the open quarters of the coat and lack of break on trousers. If we look at the other man, we see a coat that is cut with very closed front quarters and trousers worn too long. So the length of coat worn by the man is "correct" but imbalanced given the cut and styling of the garment, a situation aggravated by overly long trousers.
Elegance requires a simultaneous and all inclusive vision of balance. Its not a question of getting the coat right or the trousers but of making a harmonious vision of the two given a particular style.
If we look at an example of Windsor wearing a different style of coat, we will notice that the length of his coat has changed as the style has changed.
The DB coat worn by Windsor has once again found the "balanced length." The bottom of his DB would seem to measure closer to his wrist articulation than the knuckle of his thumb so many would say his coat length is "incorrect." But the coat length is balanced and that is what matters to sophisticated and knowledgeable dressers.
One thing I do notice among many bespoke dressers is a tendency to wear coats and trousers that are much too long. This is no doubt the result of trends in RTW fashion, and the writings of some misguided "experts." An overly long coat and trouser renders a heavy, dowdy, sacerdotal image. A "balanced" coat and trouser length is fresh, spry, agile. Study the examples of Windsor and Astaire to see this dynamic look.
Memorize it and take it to your tailor next time.