Learning a lot from this shared video, especially on the part of demonstrating the proportion. Mark mentioned that sport jacket could be shorter because they visually look longer and I also saw lots of examples from Isaia and other trendy brands tend to have shorter sport jacket. Just wonder whether sport jacket should follow the 'cover the butt' rule?It's partly a matter of style. Some tailors like to cut coats with a slightly longer front balance, such that the front of the jacket hangs a little lower than the back when you view the jacket from the side. Other tailors like the back to be even with the front. But when the back is longer than the front, things start to look off. It can also look like the coat is riding up on you at the front, maybe suggesting that you have a bit of a gut.
In ready-to-wear, you'll often find that the front balance is longer than the back to account for a gut. If you have a bit of a paunch, then the coat needs a bit more cloth to cover your stomach. This "lifts" the front balance, so that if you have a lower front balance to begin with, it ends up being even on a paunchy stomach.
Mark at The Armoury explains this concept at the end of this video.