If the flat has the same volume as the simple cup… then that’s all I would do on blank development. I haven’t run numbers, but I bet the flat is the same volume. If so then I’m positive the blank will be too big. Because like Anna said… stretching. AND, forming conditions influence stretching. Form radii, hold down pressure, lube, depth. Blank dev. is like predicting how water will run off a table top.
But at least it’s a theoretical starting point and it UNFOLDS. Cause the best part of SW’s Sheet Metal to me is that the flat can be changed.
I think what people will use the curved flange most for is making an extrusion (flanged hole). That’s one of the first things I tried and it worked. Now an extrusion flat will be way off because of sever stretching, but it unfolds. You’ll have to fire up $10000 software to guess more accurately.