Hello
I wasn't satisfied with my instinctive shooting skills either, so I tried gap shooting. I absolutely love shooting this way because theres a reference point. My groups and hitting what I'm aiming at has been great and I can compete with my son who shoots a compound with all the bells and whistles.(shooting purely instinctively, he'd blow me away)
I'm no expert on this subject, but I shoot three fingers under with a 65" homemade laminated longbow at about 53#. I use a fast flight string while using a tab. I have a three finger under tab, but I use my old split fingers tabs as well with no problems. The one thing thats important to mention is that my longbow is centershot. When I made it, I purposely built up the handle width above the grip area so I could cut the shelf dead center with the string. So I have no estimating left and right the farther from the target I get. I anchor with my broken finger just touching the corner of my mouth and my outstretched thumb resting on my neck while my dominant eye is looking straight down the shaft. Using the tip of my arrow for reference, at 20 yards the tip will be about 8 inches below point of desired impact, 6 inches below at 25 yards and right on at 30. Once I have zeroed in on the target,the arrow becomes secondary and I focus on the bullseye. All these are just factors that work for me and my longbow.
Experiment to see what works for you. And keep looking on the net for expert advice on the subject, I'm sure its out there.

Happy Hunting
Dan