Maybe you should take a look at your loading scenario (what is causing the deflection) and find a formula that lets you calculate stress caused by deflection. Then set the stress equal to yield strength of the material, and see what it spits out for a deflection value.
Steel will "lose" its elastic properties at 3% strain (or deflection) and remain bent after the load (the force that caused the deflection) is removed ... But I don't think that's what you wanted to hear.