How to calculate / incorporate a bend radius into flat plane measurements

I am not educated enough to really explain what I'm trying to do here so I'll try my best.

I am making a part out of sheet metal that has some bends in it. I know the overall general dimensions of the FINISHED part, but how do I translate a radius into my length/width measurement, with respect to material thickness, into a final dimension?

let's say I have a 2mm thick piece of sheet metal I need bent at 90 degrees, one run being 20mm , the other run being 30mm. i know the overall length of material will not be 50mm because I will lose some length in the bend. How do I find that 'slack' to make up?

thanks nerds :) I was too broke for college.
 
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