The Machinists Handbook gives .55 for Al and Brass, .64 for soft steel and
.71 for cold rolled steel. Stainless is notoriously anisotropic and
sensitive to grain direction but a K factor of about .65 should get you
fairly close. Sheet metal workers normally determine these values
experimentally based on the materials they commonly work with. This value
is much less critical with thinner sheets than thicker ones. If you don't
have sheet metal capabilities in house your best bet is to just give the
shop a drawing of the finished part and let them work out the bend
tolerances. Hope this helps.
Pat