I typically let the wood drive the shape. By this I mean the diameter of the blank Vs the thickness of it first of all. I try to use as much of the material in the final shape as I can as well. For me, the process goes like this, first, I drill a hole in the topside of the blank to attach a screw chuck. Second I proceed to true up the diameter. Depending on the blank (solid band sawed shape vs a burl perhaps) you may come close to your final shape quickly here. The third thing I do is true the bottom face and add my dovetail. While forming the outside diameter of the dovetail , knocking off the material in between to to get to it, can drive the rough shape. Now, I basically have 2 points to connect from the major outside diameter of the dovetail feature and the top rim. For me, there arent really too many shapes that fit here and are aesthetically pleasing. Depending on the diameter differnces, an ogee curve maybe or just some elliptical shape. Other times, a bad catch can also send you down a detour!. Just have fun with it.