Your elbow should generally align with your opponent’s chin. This is not the case all the time but is a good rule. This really has nothing to do with your elbow and everything to do with aligning your wrist and bicep to the opponent’s cortaid arteries.
When you put on a choke you are essentially using the positioning of a triangle with your arms. You will see this is a common theme in almost all chokes. Your goal with this is to make the triangle smaller and smaller.
Pull Back or Push Forward
When you put on the Rear Naked Choke often people will say to pull back or push forward on the head. For me I always try to push forward and make my opponent touch their chin to their chest with my arm underneath the neck so this is impossible but the idea still makes sense. Others prefer to pull the opponent back to expose more of the neck.
When you put on a rear naked choke once you have it locked in make sure to rotate your arms to the side you are choking with. This will increase the pressure of the choke