Metal stretches on the outside of a bend and compresses on the inside, so the flat blank is never the sum of the finished legs. Enter your thickness, radius, angle and K-factor and get the blank length to cut.
Notes
What the K-factor is
Somewhere inside the bend there is a layer that neither stretches nor compresses — the neutral axis. The K-factor says where it sits, as a fraction of the thickness from the inside face. Around 0.33 for tight bends and soft material, up to about 0.5 for large radii; the reliable figure is the one you measure on your own press.
Allowance, deduction, setback
Bend allowance is the length of material inside the bend itself. Bend deduction is how much shorter the blank is than the two outside legs added together. Setback is the distance from the bend line to the outside corner. Different shops work from different ones — all three are given.
Why your first part is always wrong
Published K-factors are starting points. Bend one test piece, measure the finished legs, and back-calculate your real K-factor. Then it is right forever on that machine and that material.
What this does not do
It does not predict springback, cracking on tight radii, or the minimum radius your material tolerates. Those depend on alloy, temper and grain direction.