I drew up a quick sketch and some basic tips on LiveJournal in regards to corset pattern drafting. It’s not much, but for what it’s worth:
I actually sat down with a chemical engineer friend to confirm the practicality of using a mathematical formula to determine the amount of hip-spring to draft into any corset. The final consensus was that an accurate equation would require either calculus or linear algebra, and an estimate equation using trigonometry was no better (and possibly worse) than guess-estimating.
I would strongly recommend a six-panel or greater pattern for an overbust corset, because it will more easily give you smoother curves over the hip.
No, the curves are normally not identical on both sides of a seam. The only time I make them identical is when the total width increase for each side is just 1/4″. When the curve is greater than that, you end up with something akin to a beach ball, where the seam has a ridge to it. I usually offset them by 1/4″-1/2″, but it depends upon the particular pattern.
Part of the trick to pattern drafting is to be able to look at the pieces and visualize the three-dimensional shape it will create. The human torso is not perfectly round. Most of the curve is at the side, with a little bit at the back. The curve on the spine is also a different shape from the curve at the side. In visualizing, or just looking at photos of existent corsets, look not just at the shape the corset creates, but the relative widths of the pieces and where the connect.