For under chin pull, at least use a bosal that was made to work with mecate reins, not a rope halter. Mecate reins on a rope halter, plus green horse, sounds like trouble waiting to happen. Any rein pull is going to turn his halter all around on his head, so he will be scrunching his eye with the rope in it, shut. His pull feel is not going to be direct, nor even “under his chin”, because of that halter sliding around to stay with the pull from hands.
Sorry, mixing your variety of equipment is asking for problems, since they are not designed to “work that way” if you don’t do each of the DIFFERENT training styles using the whole package correctly. Mecate reins for doubling him around depend on having a stiff bosal, the heel tied up behind his ears with the light rope, so when you give him rein signals, he FEELS it, then feels when you “let go” and bosal drops back into place. You are not giving him good directions in a rope halter that slops all over his head and face with rein pulls, no signals with a clear understanding for him to follow. You will be unable to “double him back around” as is done with the beginner horse in a true hackamore set-up. May not be able to stop horse! Rope halter in the example is like talking with your hand over your mouth, across a room, but you can’t speak louder. CONFUSING to all.
I am hoping for your sake, horse continues to stay quiet and accepting. I suggest you go back and read some of the BASIC “natural” horse trainers, not the off-shoot trainers with their own agenda and “tack”. Dorrance, and those other old guys everyone learned from, then went on the Clinic circuit.
Starting horse can be peaceful and gentle, but you need other tools than a rope halter and that rein setup.