Here you seem to make a case for a precise 90 degrees. Do you have a reference for this information?
When you're in the trophy position, the upper arm should already be coming out of the torso at the same angle where it will be when you contact the ball. ........................ Any angle above or below that mark just leads to inefficiency in the motion, lost speed, and possible physical complications.
Keep your eyes on Safin's upper arm. It remains almost locked into place throughout the stroke. His whole service motion is a study in 90 degree angles. It's almost perfectly efficient, from the waist up.
People who try to keep their upper arm and elbow at those 90 degree marks tend to rebel against it. They feel like they're not moving their arms enough. That if they do more with the arm -- bend more, flex more, reach more -- they'll generate even more speed. But every move beyond these ones just robs the motion of pace.
I used to believe that '90 degree' interpretation but I put a protractor on some serves at impact and now believe that the abduction angle, upper arm out from the side, is more like 100 d. and higher for ATP strong servers.
Federer is one that often has a greater angle. It is possible that Federer makes some adjustment for his back flexibility, but I don't know that.
Still, drawing a line between the two shoulders and seeing how the upper arm lines up to that line is a useful way to see if the alignment of the upper arm might go up too much. I don't think that it is too precise.
I have seen posters with about 90 d and it does not quite look right - see the Ellenbecker video. Remembering one poster, maybe it causes the body to tilt to the left and leg to the right?
The issue is complicated since the scapula itself, with the shoulder joint part of the scapula, moves around on the body, it can tilt. If abduction is measured from the side of the body, as it usually is, it takes some interpretation to get a precise angle. I have no confidence saying a precise angle is correct....... I can see some bad and suspect angles though.
Also, the shoulder girdle tilts and this capability probably varies considerably with servers.
Here you are not saying 90 degrees.
What angle is seen with whom is trivia.
Few if any pros have utterly perfect mechanics across the full spectrum of the bodily motions needed to produce a serve, just as few pitchers have perfect throwing mechanics head to toe. Will most of them, taken in aggregate, normalize to something like this? Yes. Does that mean Federer's does? Not necessarily. Which is why he's always based his service success on precision and disguise rather than pure top-of-tour heaviness. He's got a great serve that he uses magnificently, but which has some inefficiencies. All pro servers have good mechanics -- and virtually all of them exhibit EXCELLENT use of their kinetic chains. But that's pretty far from the same thing as having everything optimized. Does that mean we should ignore the point of the upper arm when it's used most correctly? Of course not.
The angles that optimize these things aren't statistics. They're not guesswork. When I bother to comment on something like this, it's not me taking a guess based on a clip I happen to like. I choose the clip because it mirrors perfect technique in the point under discussion. There's a reason Safin is taking 3/4 speed swings with little lower body involvement in that clip, and is still absolutely whipcracking serves. If one wants to understand how to serve well, it's worth understanding what the reasons are. Since I do understand them, I spend the time and effort to share.
It makes no sense to focus on doing anything improperly, especially when the point of doing that thing is under discussion. What I said above is how to do it most properly, and it's the point of the "high elbow." From that spot, the upper arm rotates with no superfluous motion into ESR, and is already in position to do the same in the ISR/upward portion of the serve.
Everyone should view the Ellenbecker video Rotator Cuff Injury and interpret it correctly. Available on Tennis Resources. It is important that servers don't orient the upper arm at an angle that aggravates impingement as the Ellenbecker video discusses.
Here is a thread with some comments by tennis researcher, Dave Whiteside and some other interesting references about throwing, the tennis serve and the upper arm orientation.
http://asmiforum.proboards.com/thread/2104/shoulder-orientation-overhead-baseball-tennis