I think 12" may be taller than you would like.

I just took measurements off of my kayak:

The cross section at the front of the cockpit is 9-5/8" tall (from bottom of keel to top of cross section). Inside the cross section it is 7.5" at the widest point. Overall height to top of coaming is 11". Its snug, but then again its meant to be. Since the coaming is only 16" x 20" the cross section falls right in the middle of my thighs (like the "masik" on a traditional kayak). If it was 10" forward, it would fall roughly in the middle of my shins/calves and be plenty roomy.

If you have another 10" of length on your coaming, I think you could go much, much lower than 12" unless you need the foot room. Even with a low forward deck you'll be able to sit in butt first and pull your legs in with that much coaming length.

My retrieval has a 30" coaming, and it feels gigantic compared to the ocean cockpit. Plenty of room for entry, exit, and stretching your legs while on the water. Its easier to carry 'large' items with you that you want close at hand but off the deck (i.e. in your lap). Not so easy to do with the ocean cockpit. Of course the large cockpit feels less secure than the ocean cockpit as there is a lot less to brace your legs against.

: Finalizing the last few details on my new boat. This morning I was took a
: scrap piece of wood and clamped it at 12" high and tried to see how
: much length I need to in and out of the cockpit. I settled on 30 inch
: cockpit.

: As I was altering my plans something come to mind. With a Yost style framing
: one of the frames is right under the front of the coming. Looks to me that
: in effect that frame will lower the height of the coaming by an inch to
: inch and half. Without doing another mock up I was thinking about just
: moving that frame/lengthening the coaming 2" forward. Anyone run into
: this or have a suggestion?