Here is a site that will help you calculate the allowance for bends in metal.
Randy has stated that there is a chart in the Tony Bengelis books, and I'm sure it works well. Randy has built 3 planes, so he should know.
However, if you are a Type A kinda person like me who needs exact calculations, this site will help you.
If you don't undertand Randy's advice and don't understand the listed site, please pull $20 outta your cheap butt and buy access to the builders manual.