[Rodger Knox]

The problem is how much work do you want to do with the radius dish?

You could leave the side full height when you bend it, but that would leave a lot of material to be removed by sanding with the dish. (or a plane or chisel)

You could also approximate the shape of the side using the neck and heel thickness, that would leave much less to be removed by sanding with the dish. I believe this is the most common method and also what you are suggesting.

Or you could use mathematics to determine the exact height of the sides at each point along its length, and then there would be no material to be removed by sanding in the dish. The problem with this method is that the sides have to be bent exactly right, and have to be perfectly "square" where they meet at the neck and heel. This is the way I do it because I don't use a simple spherical geometry for the top.
All these methods work, just different roads to the same destination.