I agree with the other posters. 12.5" block is a very odd size. Depending on the block itself, you might wind up adding 1/8" or 1/16" or 0.0023467" to the pieces, which would make it difficult to cut. That's why we would need to either see the block or have the name.
If you are going with the 12.5" size to fit a particular space or bed, I would consider adding an additional row of blocks, a border on the whole quilt or add a narrow sashing (1/2"), or increase the sashing than you planned by 1/2".