If it is a real seat back like in most rec kayaks, they usually tilt backwards. So relying on it at all for positioning will tend to leave you sitting too far back and not sufficiently erect to get decent torso rotation. The most effective hip to torso angle for that is usually straight up or tilted a bit forward from your core.
That is why sea kayaks tend to have back bands, which sit lower and just mostly support the pelvic girdle. The bulk of the torso support should come from the abdominal core.
Try this sitting on the ground, not in the boat, and you will find there is an optimal angle both between hips and back as well ans between hip and hamstrings where you can get the fullest rotation. Then see if you can get the same angles seated in your boat.
Canoe / Kayak Anchors
YakCatcher Rod Holder
Wall Mount Boat Racks
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