I question the 'rule of thumb' down in this thread about seat tilt, if it is based on furniture design. Most furniture puts your feet under you, rather than out front as in a kayak, and on furniture you're generally not driving any load heavier than a knife and fork -- so no reason to think the same ergonomics or geometry would apply.

I agonized from numbuttitis on my thermarest pad, then (as readers here know) I agonized over how to fix the problem. Finally, I settled on following these plans...

http://www.oneoceankayaks.com/Wshophtm/kayakseat18a.htm

...which provide *superb* instructions on how to custom mold an ideal seat. I love it, and it was fun and easy to make. (Thank you Vaclav, from the bottom of my...well, bottom!)

The proper angle of your seat surface is determined by your thigh as you sit with your feet positioned on the pegs. So, one thing I would add to Vaclav's great instructions is, I sat on a thin pad in the boat with my feet on the pegs, then slid a 3-inch block of foam backwards until it just touched my thigh. This defined how long the seat should be from the cheek cups to the forward edge. Everything else just as he says and you can't go wrong, I promise.