That's great! Both the posting and the picture, and sorry for changing directions, Lindaschipper.
I seem to be constitutionally unable to make tops that can be finished with one width of fabric, so I'm often confronted with how to get just a bit more back than what I have available. Or that I have excess fabric from the top of the quilt that I want to use up. More and more I'm putting stuff like that on the back on purpose.
Just like you did with the contrast strip, I would make the center strip something that doesn't relate to the top at all, so if there is a center seam and the blocks are based on a 3" grid, I'd make the center strip like 8" and then try and center the top along the middle without worrying much.
Being in some sort of thirds usually works well, another option would be to put on edges on both sides. I know it seems counter intuitive but you would actually trim some of the main fabric so it was obvious that yes, there are supposed to be bars of fabric here. So, instead of a 40" with two 2" strips, cut the 40" down to 36 or so with 6 inch strips.