When I setup my A/P with a full 1/4 wave radiator in the top instead of using a top hat, I started at 77" inches which was based on my estimating the length info in the manual for the AstroBeam driven element.
I did not see resonance in the CB band with the A/P top radiator at 77" inches, but I was using my antenna anslyser. The SWR was as you noted 1.1:1 across the 40 channels. But IMO that was good and was likely because this antenna is very broad banded.
My analyzer showed 77" inches was to short, so I made it longer until I got to 89" inches. I ended up with about 2" inches of overlap between these two pipes, and that is when my VA1 showed resonance at 27.205 MHz instead of higher in frequency near 10 meters.
See the image in the PDF file below. Sorry this image is set to the landscape view.