The reason this pattern works as a 12" block is that there are 6 equal size patches on each side, and 6 divides 12 nicely. To make it as a 10" block, you would need for each of those HST's to measure 10/6" finished, which is 1 2/3". That's not an easy measurement to work with. I would pick a different block. Failing that, I might print the block, reduce it to 10", and then paper piece it.