Tobus, The best answer is that it should stop where the sewn part of the pleats ends. Ideally, you would put on your kilt and get someone to measure, since you might wear your kilt differently. As a practical matter, I have just measured 7 or 8 kilt jackets in different styles, (Argyle and PC) both off the rack and home altered. They all seem to hover around 27 or 28 inches from the top of the collar. My Saxon sport coat I wore yesterday measures close to 32 inches. Those four or five inches make the difference.

The skirt of a kilt jacket seems to run about as long as its sleeves (maybe an inch more), while a saxon one comes to the cupped fingers.

Interestingly, I have found velvet sport coats on eBay, sold by ( and possibly for) Asians, that are tailored to kilt proportions. Unfortunately, they run very small, so that their XL fits about like a 40 reg, but the sleeves are fine.

Maybe there has been a gradual lengthening of the "short jacket" for saxon menswear over the years. Others have commented that group photos of Scotsmen from the mid century seem to include a number of "ordinary sport jackets" over kilts. My own experience has been that a sport coat ( or a suit coat) ought to cover one's seat and even the slightest deviation looks and feels awkward, but I am a Post-WWII product. Clearly someone somewhere likes that shorter look.

One last thought on this. Barbour, the makers of waxed cotton jackets, offer several styles, but two of the most popular are the Bedale, which is short, very like a kilt jacket, and the Beaufort, which is longer, just a touch longer than a suit coat. You see both with the kilt and both look fine, but they are not designed as dress coats.