Here's how I do a flange ... cut a strip 1" wide and fold in half to make 1/2".
Baste it to the main part of the quilt with the raw edge against the raw edge of the quilt and fold facing to the centre.
Add the next layer of border the same as you usually would.
So in essence, the flange does not add any width/length to the quilt ... and you end up with a 1/4" piece of fabric sticking out from that seam.
For your quilt, I'd add your flange to your border that;s there now, and then the next red border would be probably 1-1/2 to 2 times as wide as the one that's there.
Here's a quilt of mine with a burgundy flange inserted ...
http://www.quiltingboard.com/picture...i-t141548.html
And another with a butter yellow insert ...
http://www.quiltingboard.com/picture...h-t189484.html