Im my limited experience (aluminium and copper/nickel) you won't get a straight length by unrolling it flat - there is always a bit of 'memory' that will give you a slight bow to it. I found I had to actually bend it the other way very slightly to get it straight. And once you get into that a) it gets difficult to stop it from turning onto its side, and b) unless you have a former with a gentle curve it's tough to get a perfect result.

I got close enough (and I'm very fussy), but not 100% perfect