Walter- I've carved about 30 shells on Newport pieces that I can recall, and have measured many on period pieces,as well as having taught dozens of students to carve them. From this I can offer the following observations that may help you in your next shell project:
There are lots of variations of Newport shells. The variations are numerous in the treatment of the rosette in the center, although the basic shape on which they're carved doesn't vary much. From the rosette out, they vary a lot less.
Some have lots of lobes, some less, but if you're carving one don't use stock that is too thick. This is the biggest mistake that beginners make. Clock shells tend to be 5/8 or less and chest and desk applied ones around 5/8 to 11/16. Most are contained within an arc, but some on secretary draws I think will fit inside an oval curve.
Make the transition from the high point to the contact with the ground at the top a gradual descent. Many shells fail due to a precipitous drop "over the edge" to the ground, which I have never seen on a period one.
The lobes are not as radically s-curved out from the center to the edge as you might suspect. The fact that they are carved into either a "cookie" for the convex ones, or a "dish" for the concave ones exaggerates the curve quite a bit and usually only the bottom 4 or so are not laid out in a straight line from the center.
Don't underestimate the time involved preparing the basic shape of the blank. Your finished shell will suffer if it's not perfectly true before you start to carve the lobes.
Lastly, rifflers, dremel tools and rough sandpaper will not help. You need to very carefully figure out which carving tools will give you the shape you need and use them. Do your final smoothing with scrapers or very fine files, then very fine paper, 400 or so. Going to the paper too soon will only result in smooth badly shaped work.
There is a lot more variation within the form that has been discussed, including the number of lobes, the scroll cut around the concave ones and the presence or lack of fillets, but if you can, look at some originals. The thing that's amazingly consistent, however, is the quality of work in these things. I can't remember ever seeing a poorly carved one on a period Rhode Island piece, which means there's a pretty high standard for us moderns to aspire to..........Al