A couple of related threads:
I’d recommend thinking about adding some features to your dies to locate the part and the dies relative to each other. 3D printed plastic pins turn out to snap easily, so I ended up using steel dowel pins and 3D printing holes on both sides to receive them.
Joe’s right that the steel will spring back away from the bend, so you need to exaggerate the angle a bit. I’m not sure how you’d do that with a U-shaped die, pressing into the U. You could do it in two bends, one to rough the angle pressing down into the U, and a second to bend to the final angle (3 part die?).
[Not a mechanical engineer!]