That won't work. A ten inch block won't easily translate into a 12" finished block without some strange measurements. It's much easier to change a 6" finished block to a 12" block because six evenly divides into 12 and 10 doesn't. It's also about how many pieces are in the block. Just adding 2" on each piece won't work as you need to allow for seam allowances. It's more complicated than you would first think.