Anchoring with the index finger in effect lowers the rear sight and decreases the gap. Anchoring with the middle finger raises the rear sight and increases the gap. So, if you switch from the middle finger in the corner of your mouth to the index finger in the corner of your mouth, the gap will decrease, regardless of the shaft material used.
If you're conscious of the gap, and from the way you described your shooting in your initial post, you are, you are gap shooting, not using split vision. Nothing wrong with that, of course. Gap shooting works particularly well when the distances are known and the gaps can be locked in. In hunting situations, where the shot distance can change by the second, split vision accommodates the changes more quicky and easily. Might want to practice that as well. Gpod luck.