Try to keep an eye on feeling might be too difficult, since feeling is a very subtle phenomena. By feeling I mean the unpleasant, pleasant or neutral aspect of sensations. What to watch during reading or listening is dependent on your present mind states.
Practically speaking: if mindfulness is sharp and there is a lot of clarity in the mind you can pick more subtle states like feeling (pleasant, unpleasant, neutral) or mental states (like sadness, boredom, happiness, etc.). If the mind is more easily distracted, there is much restlessness, excitement and such you might want to keep an eye on more coarser things like the bodily sensations, the bodily postures or coarse hindrances, etc.. Because to be able to watch more subtle things the mind has to be settled down.
So, my advice would be to adapt what you watch to your state of mind as well as the present situation.
Hope this helps.
