I think you are pretty close or your bare shaft wouldn't have consistently been in there with the fletched shafts.
I'd guess you are still slightly weak and if you could move back to 25-30 yards you might find your bare shaft impacting slightly right of your fletched shafts. I'm one that doesn't pay a lot of attention to the angle of the bare shaft, but theoretically the bare shafts should be in the target at the same angle as the fletched.
I will say that I like to shoot more than one bare shaft. A nock that is even slightly out of true is enough to through off the flight of a shaft without feathers.
Rather than worry about angle, I just move further back, although I think for most of us anything beyond 20 yards is fine tuning . When I get to 25-30 yards if the bare shafts group with fletched I call it done, even if the bare are slightly angled in the target. At that point I have never not been able to swap the field points for same weight broadheads and get perfect arrow flight.
Lets face it, bare shafts are very sensitive. Thats why they work so well to tune your bow. You can drive yourself crazy trying to get the perfect angle when the truth is close enough is usually close enough, at least in my opinion.