Do you have long arms and legs compared to your torso? Honestly, the jacket fits to the extent it covers your crotch and buttocks - so I think you are fine and meet that "rule." But it does seem a bit short which makes me think you have disproportionately longer arms and legs. If that's the case then it may fall a bit short of the other "rule" that the length of the jacket is about half the length of your body from the neck down. Anyhow, I wouldn't stress over it at all - I respectfully disagree with Richard's opinion that it is too short by contemporary standards - indeed, I would say it falls well within contemporary standards. Most modern fits don't cover as much real estate as your jacket covers. None the less, for your next suit, consider at least an inch longer. I will add that I like my jackets longer than is currently fashionable which keeps me away from some RTW and would probably get me in an argument with some sales people over what is a good length for a jacket. Reminds of one RTW jacket that I had altered where I was told "never buy a jacket that long again." I think it might be an inch longer than most of my stuff!
This is a custom made suit, and appears to have a little fabric inside the back. Would it be dangerous to ask the tailor to lengthen it by a half inch? Is that possible and would it screw with the balance?