Believe it or not, this is a complete overhaul and re-envisioning of this (I also added a pic at the bottom)

A quarter piece is “cut out” from the yo end, mirrored book-match style and the 2 halves then joined on an angle. for a winged V shape.

I tried a more subtle look in (non striped) Stockinette but it insisted on curling no matter the style or width of the edging. Garter stripes won the argument. The stripes are successful in accentuating the offset curvatures and are nice and graphically bold for pictures. I like it! I’m upgrading from Swatch/Prototype to Sample 1 - pattern to come (someday)

Two mirrored pieces - one built right tip to center (k2tog), the other left tip to center (ssk).

The yo increase stacks along the same column, moving across by virtue of the upper edge decrease. On the row the decrease takes up the yo into the upper edge, a new yo stack will (in the same row) begin anew at the lower edge.

Each stack set doubles the # of yo it takes to reach the opposite edge 1, 2, 4, 8, 16, 32, 64.

YO on every RS Row. But only decrease 1 (k2tog R half/ssk L half) along upper edge every other RS Row (the stripe reminds which row to skip /work = Dec on black rows work even on multi.).
The overall st count inc rate is 1 st per 4 rows.

After the final yo of the 64 segment the yo is not re-established at the opposite side, instead it is worked with the established Decrease only, for 9 rows creating a center back notch.

Then the piece is put on hold while making it’s mirror.

The two pieces are aligned onto one needle and then center double decreased on every WS row, giving the 90 degree angle for the center spine. For a less deep/more curved back it can be worked at more rapid decrease rate, with CDD on both RS and WS at the center line.