Hey, welcome back.
That's an interesting way of visualizing it. Obviously we'd need to know a and b already but I can see that potentially leading to something.
Is this an accurate diagram in terms of the numbers, or did you just do that to visually show where everything goes with arbitrary values? Because if it's accurate, just thinking in terms of symmetry, the diagonal from the outer corner of f to the corner of the center square (so not dividing the f box in two, just a diagonal through the squares) is 7 boxes and so is from the outer corner of nn to the inner corner of 2d(n-1). And 7 is prime. I have a feeling this is just arbitrary values though.