In my experience, having outfitted numerous solo whitewater canoes and kayaks, and from talking to the most experienced of the local packrafters with more whitewater skills than I (Class IV WW Solo Canoe) the most efficient paddling seat location is as far forward as possible. This allows one to paddle with the modern kayak stroke.
Ideally the paddler would be in the center of the packraft. Like a kayak seat. However, this is too far forward for most packrafts. John and others want at least 8 inches (20 cm) between their backband and the back of the kayak. It’s a fine place for day gear in a 30 litre dry bag or small pack.
My favorite set-up is the DIY seat, with a back band preventing my Crazy Creek chair about 8 inches forward from the back of my V3 140. I put a throw rope, day bag / snack bag behind me. I don’t have room for anything at my feet.
For more about the stroke try Ken Whitting: https://www.youtube.com/watch?v=F2fGKNFEURw
Perhaps even better is Eric Jackson: https://paddlingmag.com/videos/eric-jackson-kayak-forward-stroke-video/
The “modern” stroke is what happened to kayaking and canoeing when they became Olympic sports. My old low paddle, bent arm stroke is obsolete and deemed inefficient. Habits being hard to unlearn, it is just so much better to learn it right first! My observation: If your arms are tired, oops, correct your stroke.
Notice in these videos that the paddler is pretty much in the center of the craft. When outfitting a solo canoe the rule is for the spine to be at the moment-center of the craft. Yeah, the spine, which is pretty far behind my belly button! Canoeist, however, don’t have legs out in front like in a kayak.
Enjoy!
Bruce Campbell