I'm not too sure I agree with all your good examples... To an extent. They all do have decent ls joint placements, but a couple look like they have a slightly weak loin.
When I look for the strength of the back, I look at a couple things. First is the placement of the coupling, or the ls joint. Next I find the beginning of the back (I find many have problems here, often confusing themselves when the wither is large and set back) and the back ends at the ls joint. We can then easily see weak backs when the ls joint is poorly placed, simply by length of back.
But the way I was taught, you must also look at the quality of the loin, which is directly related to the ls joint placement, but there can be weakness here too.