IMHO I would go all the way back to the narrow boarder (before the prairie points), because it looks a little more wavy than the outer boarder. I know that is a lot of work, but in the long run you will be MUCH happier.
I also had not heard of deducting a 1/2" off my the middle measurement. But, when you think about it.... hmmm couldn't hurt. Here is what my simple brain figured. When sewing, the quilt top is on the bottom, the boarder on top. Okay.!! The sewing machine will take up the difference, because the feed dogs will move more fabric, the boarder is just along for the ride.
The other reason I can see this working is that my first quilt instructor said that if you have two blocks that are not quite the same size (but should be) put the larger block on the bottom.
good luck unsewing