I don’t think there’s a single answer as to curvature. It varies for sure and you’re right about the angle at which the sleeve exists the shoulder. This plays a role too. I can’t give a single answer and I haven’t taken something apart to make the detailed notes required because at that point…why not just buy a jacket you can slightly modify without reinventing the wheel? The argument for and against 5 Star is circular. It just keeps going on and on and the end result is always the same. In other words…..Simulpost!
I'm not saying the darting (don't have a better term yet, and that's what I've been calling it in my head all week) is necessary, just a method that I have in front of me that adds both curvature AND dimension. Personally, I also like the detail of it for the style that's in my head.
It clearly IS down to how the material is cut, but HOW is the material cut? What is the actual difference? This is where people with sleeves that are "good" would take detailed photos where individual panels of sleeves are made as flat as possible. I suspect, there is curvature in the cut of these panels, and that's all there is to it. What is the angle though? Is it a gentle curve along the entire length, or are is it straight at one end or the other with a curved section (X%) in the middle?
To wit, here's an image of a jacket pattern where you can clearly see curvature in the arm panels (the panels themselves down in the bottom right corner parts E and F, not shown in the macro views at the top):
I picked that one specifically for the curvature of the actual panels, but there are plenty of jacket patterns out there with clearly straight arms.
Additionally, in the interest of putting things into finite terms for entering into the Fivestar machine, what ANGLE are the sleeves mounted to the body of the jacket? Maybe relative to the side panel (or is it too variable according to waist/hem dimensions)? The angle of the shoulders probably matters as well; what is that angle from some imaginary center line? To fully code a jacket, these details need to be known, otherwise we get whatever the pattern guy on hand puts out.