Reducing 12" down to 8" will be difficult to measure. It will be 2/3 in size, and made with 4 x 4" units. Two are made with hst's and squares. I figure these are roughly 1 5/16" square, that's not exact either, and you would have to add seam allowances.
Could you not settle on a 9" block, or a 6" one with a 1" border Maybe someone else has a better idea.
Good luck