I know the question is about deck height but other cockpit dimensions are equally as important for other reasons and can be interrelated. Bryan and Duane always give great advice while I over analyze everything and add minutia. I am long legged (I am 6-2) and like 4 inches behind my back so my keyhole cockpits are 33 x 16 inches. The cockpit length will need to be longer for a low front deck than a high front deck. Cockpit widths tend to be too narrow. The coaming lip width should be as wide as the kayak’s shear for a opening that will allow the paddler’s butt to get as close to the shear as possible during balanced braces, Petrussen maneuver and hip snaps. This will allow you to get the hull under your butt before bracing up (head last of course), a low deck at the back of the side of the coaming also helps. The back of a keyhole coaming should be a half circle because it is much easier for your skirt back to stay on while stretching the skirt over the front of the coaming. This well appreciated in rough seas when white caps are washing over the decks. The knee braces should secure your legs behind the kneecaps (not your thighs) so not to pin your butt to the seat when lying back. And your seat needs to be as thin as possible at the low spots (butt bones) to lower your weight to stabilize the kayak.