Dave,
I have sort of evolved out of pure instinctive shooting myself. I find that pure instinctive shooting works well for me at the distances I can practice at daily. I use instinctive shooting when I hunt, because I won't take a shot unless it's within my instinctive shooting range.
But for longer shots at targets, I need something different, maybe because I don't get to practice the longer shots daily, so I don't ever develop an instinct for that kind of shot. What I find works for me is to include the tip of the arrow in my awareness. I don't measure a gap, so it is not the same as shooting with sights. I focus on the target, with the arrow tip in my peripheral vision. The difficulty is in maintaining my focus on the target, while being aware of the arrow tip. If I let my focus shift away from the target, the shot is blown.
For 30 - 50 yard shots, my normal split-finger hold with my middle finger in the corner of my lips, and the bow canted at my normal angle, places the arrow tip either slightly below to slightly above and to the right of the target. For longer shots, I use my index finger in the corner of my lips.
I don't try to guess the range, and I don't try to set a gap. But being aware of where the arrow tip is is more information being fed to my mental "computer" and definitely makes me more accurate on longer shots that I don't have a chance to practice very much.
It might be an alternative for you to try if you don't want to switch to gap shooting, but find pure instinctive shooting inadequate for your needs.
Dave