sewbizgirl

What I do is look on the back first, to see if I have any big seam allowances. If I can 'let out' a seam and still have a scant 1/4" left, I do that. But if the seams aren't big enough to let out, there's nothing much to be done but to add a small border all the way around the block, and then cut to 12.5" with my 12.5" square ruler. Even if the border comes out very small, it's better than sending a block that is less than 12.5 and won't match up with all the other blocks in someone's quilt.

But the thing to figure out is, why are my blocks coming out small? Consider the seam width you need, and the weight of your thread. Fat thread takes up more space when the seams are pressed to the side. You would need to adjust down to a scant quarter inch seam. You can do some practice seaming with scrap squares, and press them open and measure them. For instance-- two 2.5" squares have to come out to 4.5" when they are pressed open. Play with that until you find exactly how wide a seam you need to take, in order to get that. Put a guide tape on your machine bed, if it doesn't have an accurate line there already, or a good scant 1/4" foot that you can use as a guide. If you cut accurately, with the ruler line on the fabric edge, and keep that exact scant 1/4" seam allowance, everything is bound to come out right. You should only have slivers to trim when you finish.

Some people have better luck using a 50 weight (very fine) thread in piecing. 40 is normal, and 30 is 'fat'.