For some time, I've known that the length of off-the-rack or ready-to-wear sports and suit jackets was not quite right on me. The length looked off for some reason. The problem originates in the practice of stock or standard patterns and sizes that make ready-to-wear (RTW) possible. All RTW garments are cut off of standard patterns that are based on the "average" person for a given size. The problem is that very few exactly fit the average model.
In my case, I find that the jacket length of most stock sizes are too long for me. So what is the appropriate length of a jacket? I wish I had read this AskAndy thread on coat lengths years ago. The basic rule of thumb is that the back of the coat should cover the seat of one's pants. There are two other, less reliable guides: (1) the jacket ends at the thumb knuckle and (2) the jacket is half of the distance from the bottom of the neck collar to the ground.
Case in point: I was standing in line recently at Walt Disney Hall for tickets to a concert featuring the pianist Lang Lang and Stravinsky's The Rite of Spring. I saw one gentleman walk up wearing a classic outfit - a single-breasted navy blazer with medium grey flannel trousers. The only problem was that his blazer was far too long, extending below his fingers. Needless to say, it disrupted the entire look and balance of his ensemble.