I wouldn't use the whang procedure on the doubler. The bow it over your thighs until it is straight. Because it rivets to the rear vstab spar, which is then riveted to the ribs and skin, it doesn't need to be perfect.
I don't think I looked for 1/16 on this part, just got it lose and cleco'ed in. I've heard of builders just clecoing it in without shaping it and being fine too.