The best thing to do in easy situations, imo, is as Randy said... draw it in the flat, then slice at bend lines, and fold yourself using rotate.
Depending on gage of material, type of process, quality of tooling and a hundred of things, your bend radius will change.
If this is 14ga or smaller, I'd say that in most cases it's safe to use the inside dimensions of the final part as your blank length. That will give you the solid to draw flat... then cut and rotate about the bend line... then you can simply create a new solid of a quarter cylinder and union it to the parts at each bend, trimming where it looks good (who really cares what the bend corner edges looks like... you'll never get the compound angle right anyways, because depending on the material and manufacturer, it'll pooch out different anyways)
What I'm wondering is this... because I've seen it a hundred times in even my short time doing metal fab work and metal fab design. After this is formed, do you want that angled side to form a nice planar surface? Because it won't.
Now, like I previously said; what Randy advised is best in easy situations where you know the blank. What I'm thinking is best for you is to model the final part, and work exactly backwards from my first instruction. With the model of the final part, slice away the corners (One horizontal slice and one vertical slice to remove the bend itself) so that you are left with plates that can be rotated back into the flat shape.
That will give you an ACCURATE blank of your part.
