I'm just finishing a college course and only had a few weeks to make something so decided to just get on with making something and not waste time designing, so a standard project with rod already drawn was a little faster. The only part i changed was the back and top rail. I think you maybe right and those rails should be higher, although there is one more component that dovetails between those rails i have yet to complete.
Sean, for what my opinion's worth (not a lot), my initial reaction to your curve placement was that they look a bit too close together (from an aesthetic viewpoint). I'm also wondering if they might dig into your spine at the point where they almost touch - there isn't room for your backbone to nestle in the gap. Just a thought, and a bit late too.
I fitted the last rail at the bottom and found it incredibly difficult to hold the chair while i cut the dovetails :? Just couldn't seem to get it to stay still while i chopped them.
I made the seat from 9mm (i think) MDF, with a piece of foam glued to the top and 2 layers of wadding on top.
On top of that was some calico.
Then came my brown leatherette.
To tidy the underside i used a piece of hessian, this also helped in giving another row of staples through the leatherette.