Sorry, I didn't read closely enough and so didn't recognize your intended result. You could just continue to add red strips until you get almost to the right size, and then add the strips to the sides that will move the white into the center. If you have precut your strips this might mean that some of them will be the wrong size, since you won't be using the normal progression of the log cabin, but after the blocks are finished it won't show up as a mistake since the eye doesn't distinguish the seam lines within the red.