Wow, how often does this come up and how useless are answers to it, at least in helping the questioner.
Here’s the problem, aside from a few geographical landmarks, well maybe one, and a basic aesthetic principle, if you ask SF about this, you’ll end up with varied answers mostly because people are using different standards and not telling you what their standards are. Further, photos are going to distort things as well. You’re polling in the worst sort of conditions.
You have to figure it out for yourself. What follows is advice and not a set of rules.
[Disclaimer: I'm not an expert. It's warm today. I'm bored. And I just saw this question asked again]
Advice 1: Cover your ass.
A good rule for life in general, but certainly for tailored menswear.
Practical advice: Find the fork of the trousers. If the jacket hits there, then your ass is covered.
Advice 2: Advice 1 (just to reiterate)
Advice 3: Aesthetically, lengthen the leg line without lengthening the torso, aiming for a vertical balance between torso and leg line.
Bisection is a good place to start, but you can move up or down using Advice 1 as your anchor point.
Practically: You have to look at lots of pictures of others who you think strike the right balance (also look at good examples in person). Look at lots of examples. Really. Dressing well is partly about learning to look.
Here's one of me, with the jacket in the range I like:
See if your jackets emulate that. In a store, try an L, an R, and an S. Take pics of each. Ask yourself: which do I like? Does it strike a balance I find aesthetically pleasing between the top and bottom half while covering my ass.
Asking even knowledgeable people to tell you on the basis of a picture you upload to SF, taken at different angles, is likely not going to help you avoid asking the same question again, probably with different responses. A never ending cycle of spinning your wheels.
Advice 4: Ideal length is a range, not a single value.
Don’t get fixated on a specific length. Don’t be an idiot. If it’s 0.5 inches longer or shorter, that’s ok. Really.
You might want something shorter for casual wear, something longer for more formal situations. You might vary for seasons, for material etc. This is the beauty of menswear: tasteful variation.
Advice 5: The Rule of Thumb
This is probably the most used and the least important “rule”. I am surprised how often people assess length from a torso shot. They are presumably using the “rule of thumb” or talking out of their ass, which ought to be covered.
Here’s what I think: arms vary in length, their relative position to landmarks on the body (e.g. the waist) varies, etc. This rule might be statistically successful in helping folks avoid way overshooting their specific sweet spot, but it won’t help noobs understand the point of finding the ideal range.
That said, on average, I personally find jackets that go beyond the thumb to feel too long for most in that I can’t help think that they are like a schoolboy whose parents bought them a jacket two sizes too big in hopes of having the boy wear the same outfit for two or three years. But that’s an expression of an aesthetic and personal pet peeve, after assessing for coverage and balance, not enunciating a rule.
Advice 6: For well-meaning StyFoDudes...
The best way to help someone, if you feel like you must answer the above question, is to explain why you think its just right, too long, or too short.
Ok, fire away. Disagree, agree, add, subtract. Heck post pics. Ask for advice.