@RogerWood I don't believe is wrong, just has a assumption which was not 100% accurate, but as comment later, it was coincident with the small-angle approximation at the end so it was interesting for noting that the anzat I am asking leads to the same results

@RogerWood specially since a pick my ansatz without no physical meaning, but as a mathematical mechanism with a possibility of fit because of its properties

@RogerWood Hope you reconsider to include it, maybe focusing it in the small angle approximation where it looks to be right