The countour roll tool (Inventor 2010 and beyond) has the ability to specify the developed length.
what kind of part are you making?
can you provide a simple example file?
Contour Roll with the Developed Length option is what you want.
see attached example IPT (2010 format) for how to set up the sketch
What I'm looking for is I want Inventor to tell me how much stretch i would get rolling a 10ft plate to a certain radius. Or if that is not possible how would you got about calulating the stretch from the Kfactor parameters?