Sashing is just a little more complicated
Although people have answered your question, I thought of something that might change what is being suggested. Your question about how to sash a block to bring it up to 12.5 inches has been answered, but in an overall quilt design you much keep something additional in mind. If you add sashing to every side of the block, when you sew the blocks together you will end up with double sashing.
If you are dealing in just sewing blocks together, you can just decide on a size and go with it. If you are talking about sashed blocks it is a little more complicated. The blocks and sashing are alternate rows. If choose to have an outer sashing, you have one more strip of sashing than the number of blocks. If you only have inner sashing, you have an extra block. You have to take that into your calculations. You can use the 3.5 inch sashing. Then every block and it's surrounding sashings will equal 12 inches finished. Just know that you will have one extra sashing piece at the end.
I have attached some pictures to show what I mean.