The rabbet needs to change bevels as it progresses along the keel from stem to stern. The easiest method to get the bevel right on a small build such as this is to eye ball it!
The rough cut should be made about 1/2 an inch proud of the actual rabbet line. At this point, a batten can be placed on the frames to see what angle the planks will reach the rabbet. With this angle in mind, adjust the plane to this similar angle and give it a few strokes, then test it again.
If you can see light under any part of the batten, you need to do some more trimming. Light between the frame and the batten means that the rabbet is proud and needs to be trimmed. Light under the rabbet means you trimmed too far and need to trim the frame a bit to compensate (this should not happen though as it means you trimmed too far).
Once there is no light under the frame and the rabbet, you are done! Now you move to the next station and adjust the bevel. Once all the stations are done, you will need to blend the spans between stations. Resting the batten on the chine log works well for these parts as it serves as a point of reference.
Once the bevels are all established, the rabbet can be cut into the keel by resting the rabbet plane on the bevel and sliding it along the keel, cutting in a nice and clean notch that flows with the bottom of the hull.