With so many asking, excuse me if I just start at the beginning.
Top Row- Imagine a tube viewed from three different angles; perpendicular, 45%, and parallel to our view. The tube does not change length, it only appears to because of perspective (things get smaller as they recede) and foreshortening (perpendicular view-long; parallel view-short) Let's call them: T1, T2, T3.
Middle Row- From the side and top we see the hands-on-hip position pushes elbows back not out to the side. Therefore, in a front view, the arms are angled back and drawn with T2s not T1s.
Bottom Row- Rotate the body to a hard 3/4 (7/8?) view. What happened to the elbows? They rotated the same number of degrees. The front arm is now perpendicular to our view and thus drawn with T1s. The back arm (if we could see it) is parallel to our view and thus drawn with T3s. Note the rear arm being hidden is irrelevant. Draw it anyway! If you don't you'll never nail the hand.
Top Figure- When doing jumping jacks, arms perpendicular to view, elbows follow a circular path (blue); Arms are drawn with T1s. Hands-on-hips uses T2s, therefore the elbow follows an elliptical path (red)
What's an ellipse? A circle in perspective. The more perspective applied to the circle, the narrower the ellipse becomes.
Bottom Figure- All arms are T2s BUT the front arm approaches T1 (wider ellipse) the rear arm approaches T3 (thinner ellipse).
Note an elbow/humerus arc is the same thing. In the example on our left, I simply didn't bother drawing the humerus but, as mentioned above, just because you don't see it doesn't mean it's not there.
Where do you place the elbow, forearm, humerus arcs... wherever you darn well say so, you darn well say so-er you. Seriously, that's the rule in perspective; two arbitrary decisions with everything else carved in stone. I decide the red and blue arcs are true (and they are because I said so, spppluuurrrrb!) THEREFORE, where those arcs meet is the elbow (that's the carved in stone part)
How did I decide these things? By thinking 3D. The humerus is about 1.6 heads, the forearm about 1.3 heads, when viewed from the T1 position. At these proportions arms can be at the wrong angle but never the wrong length. As our view changes from T1 to T3 both measurements approach 0 heads.
What angle do you want? The farther back, the greater the angle, the narrower the ellipse (and more thrust the chest). Relaxed, the arms are approx 90º to each other. From a front view it would be 2 ellipsis of 45º or - 90º (ta dah). Rotate the figure so one arm uses a 55º ellipse and the other will use a 35º ellipse for a total of - 90º (math, gotta love it)
If I can mix my sci fi movie metaphors, "Use the force Luke." The point of learning to do this is so we don't have to do this. Commit this type of 3D thinking to your bones and forget about it. Thing is, you can't forget. Not if you really learned. It will always be back there.
3Dville cannot be some foreign destination achieved by plane, trains, and automobiles. 3D thinking has to be where you live, eat, sleep, play kissy-face.