1. A computer implemented method for utilizing custom equations for the unfolding of sheet metal in a solid modeling application, comprising: obtaining a drawing model comprised of a sheet metal object in the solid modeling application; displaying an equation editor user interface; accepting, in the equation editor user interface, user input that custom defines a custom equation for a first variable that is directly used to calculate a developed length for a bend in the sheet metal object; converting the custom equation into a standard equation that produces the developed length and is accepted by a modeling kernel of the solid modeling application; the modeling kernel dynamically displaying an unfolded version of the sheet metal object based on the standard equation and the calculated developed length, wherein the standard equation is not exposed to the user.