Fernando, The sheet metal rules are based on several factors, but the two most important are the type of material and thickness. When sheet metal bends, the inside of the bend is compressed and the outside of the bend is stretched. The neutral line is the point in the thickness that remains the same length. To calculate the bend allowance by hand you have to know where that neutral line is. In order to create a flat pattern (if not using a CAD package such as Inventor) a designer usually refers to a bend allowance table. For the most part, if you choose the correct material and thickness, Inventor will correctly create your flat patterns - to within a few thousandths of an inch! Here is a link to a resource that gives more information on sheet metal unfold rules: ketiv.com/wp-content/uploads/2019/04/KETIV-AVA-Sheet-Metal-Unfold-Rules-Equations.pdf