Arm carry is highly individual. And your default elbow angle may vary, depending on distance: My middle-distance carry had an elbow opening of ~100 degrees; my distance carry was probably closer to 80. (I too was ~6-3, but average leg length for that height.)
The real key is that there should be some "play" in the elbow, at any running speed. This is obvious with sprinters--as their elbows swing back they open to ~140 degrees, and as they swing forward they close to less than 90.
https://upload.wikimedia.org/wikipedia/commons/thumb/e/ef/Michelle-LeeAhyeHeat3Of200mJORio2016.jpg/250px-Michelle-LeeAhyeHeat3Of200mJORio2016.jpg
This opening and closing of the elbow will be much less apparent at slower speeds, of course, but even at a jog there should be a little bit of it. Locking the elbow (at any angle) forces twisting and/or sway of the upper body, which wastes energy and reduces your speed.
[Curiously, letting the hands flap at the wrist--in an effort to "stay relaxed!"--can end up locking the elbow. Relaxation at the elbow is important; relaxation at the wrist, much less so.]
It can also be helpful to understand that only the *backward* swing of the arm helps to move you forward; the forward swing is a recovery stroke. Having a focus on the backward swing of a relaxed elbow can help you develop arm-form that is optimal for you.