The “rule” that an informal upright tree should have the apex over the nebari is it should visually appear to be over the center of the nebari when viewed from the front. When viewed from the side almost every high quality informal upright will have the apex quite a bit in front. Usually to the point of where the apex is actually above the rim of the pot! If the apex is even farther forward than the rim, it looks funny.
The only styles where the apex is “actually” over the center of the nebari is formal upright, and broom.
The whole reason for the forward lean has nothing to do with the tree “bowing” or being friendly (although those explanations seem to make sense of why forward leaning trees look better than trees leaning back), the real reason is the forced perspective the forward lean imparts. It helps with the illusion. And, the tree actually appears SHORTER. With the tree coming forward at an angle, the same length of trunk would be lower than if it were stood straight up tall. If the degree of lean is more up near the apex, it also makes the internodes (the distance between branches coming off the trunk) look as if they are closer and closer the nearer to the apex. It’s an optical illusion that our eyes and brains interpret as compressing the tree. (This works so well that most of us aren’t even aware of it.). This also explains one of the reasons a true formal upright is so difficult to do. We can’t trick the eye using foreshortening to make the internodes appear to get closer up near the apex. They actually have to be! It’s very hard to achieve! Which is why you rarely seen a really good FU.
I’ll illustrate my point with my JBP:
As seen from the front:
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The right side:
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Notice on the above picture that the apex is forward of the nebari, about over the soil near the front rim of the pot. The front is rather flat, but you can see the back branches extend quite a bit farther back behind the pot. They are substantially longer than the branches on the front or sides. They provide depth to the image. You will also note there are many more “back branches” than “frontals”.
Here’s the other side:
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As you can see from this side, the lower trunk actually moves towards the back before moving towards the front about 1/3 the ways up.
And the back:
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Lots of branches, lots of foliage, these provide “depth”, and frame the view of the trunk from the front.
Now, go back and look at the front. Does it look like the front leans forward? No, it doesn’t. But the front lean certainly helps the image.