You can select an edge as reference geometry for the unbend.
Do this early in the design, setup>fixed geom.
It is possible to make a part in sheetmetal that you can't unbend, I hate to admit it but I have done it.
The form feature you have to do an unbend form which doesn't give you developed length.
If you find you have to, you can cheat by putting in a small flat surface and use that as the fixed geometry in a pinch, hide it in the drawing by deleting the dimensions for it.
jperkins