Try this way- start with just an arc(see doc1)
After desired sketch select base flange(sheet metal feature) see doc2.
Set parameters(k factor=.428"),thickness(t=12mm in your case)
After setting - click ok.
From tool bar(must add to tool bar) select flatten - will give you flat.
In order to add holes in flat you have to unfold first,add holes,fold.
I don't use sheetmetal much at all (as is probably obvious) but marks method worked for me. I will try the other methods here too.
thanks for all the info!