i agree with your premise on the light square having to be cut smaller then the dark square because that has to be cut according to triangle math of 7/8. i think the author addresses this in her pattern and thats why i asked does she start with a square and have you cut it into 4 triangles.
i don't know why you'd want to have one of the outside corner triangles off from the seam line.
how would you sew that triangle section into the next block when the edge isn't even with everything.
maybe you're thinking i want her to make a perfectly equal 4 patch and of course its not because the dark section will be bigger than the light.