Your arm positions should not be affected by what figure you are dancing - they are not handles to torque your body with. Until that habit is broken (I remember it took a while) there's not much point in worrying about where your arms are, as it's going to be a constant struggle. But once it is broken, keeping the right arm position is not much harder than finding it in the first place.
However the drill you suggest results in a fautly arm position. You must not have your elbows in line with your body - doing so will make the shoulders cave forwards and create nothing for the man's right hand to connect to. The elbows need to be slightly in front of the back of your body, so that your elbow-back-elbow forms a gentle curve. If you want a drill for the right position, put your fingertips on your sternum.
Note that most men have their left arm far too forward - if this is the case and the lady puts her right elbow in front of her back where it belongs, the hold will be very open on that side. The fix is for the man to bring his elbow back until it is just in front of his back, definitely no further forward than the lady's!
Commonly the wrong position gets set up as the hold is taken - the man puts his hand up, the lady takes it and then rushes inside of it - and thus inside her own frame. She needs to stop moving forward on her own the instant their hands meet, and come into him only as he draws his left hand back to its ultimate position.
