When cutting seats in prongs I use a setting burr the size of the
stone or slightly smaller and cut all the seats at once. These are
the burrs with a normal pavilion shape on the bottom and vertical
sides. I rarely use a burr that is significantly smaller than the
radius of the girdle where it meets the prong. These seats are
easily modified to suit deep bellied stones with bud or ball burs.
With thick girdle stones I will usually use a bud bur. Shaping seats
can be done with many different tools. Specialized setting burs,
ball or bud burs, files, gravers, drill bits, separating discs,
sanding discs, pretty much anything that will cut metal can be used.
The more flexible your mind the more options become available. Look
at your stone from all different angles, look at you tools without
prejudice and try to relax! Also, if a prong seems reluctant to move
the way you think/wish it should, often trimming the outside of the
prong above the girdle can help facilitate progress.
Hope everyone has/had a peaceful harmonious Thanksgiving!