Just to throw another idea out there:
Another way to look at rocker is to compare the lateral plane area to a rectangle with the same waterline length and draft of the boat. The lateral plane area is the area below the waterline of a plane on the centerline described by the the keel line. If you take a ratio of the lateral plane area divided by the LWL*Draft you will get the lateral plane coefficient (Clp).

A boat with a straight keel and plumb ends will have a Clp of nearly 1.0, a boat with a lot of rocker will have a Clp closer to 0.5. Higher indicates less rocker. This may not be as intuitive for some people as rocker, but it is really giving the same information. To get an idea of the Clp of some boats you can look at the particulars of my designs at: http://www.guillemot-kayaks.com/Building/Guillemot/study/index.html (table at the bottom of the page)

Mike is correct that the depth of the rocker is important for estimating things like maneuverability. But the depth is just the draft. I think it is reasonable for designers to supply the draft by itself. I don't think it is required that the rocker include the draft in it. To really get a feel for the real bottom shape it is reasonable to look at both the degree of curvature as well as the overall draft.