Anything smaller than a 1" border is a real pain when you go to join the blocks. I hope you have enough seam allowance to let your seams out a bit. What I do is sew a second seam just inside the first one, and then pick the first seam out. Cumulatively that should give you the 1/4". It doesn't take much.
And ditto what Janice said about not measuring on your mat. You need to put an acrylic ruler on top of the block to really tell. And start measuring from a corner of the ruler so you can accurately tell if it's small. If you don't have a 12.5 acrylic ruler, you really need to buy one if you are going to be making 12.5" blocks. JoAnn's has them either on sale or eligible for a coupon just about every week.
Good luck, Pamela! Let us know how it goes.
edit: Oops, just read that you do have a ruler and have pretty much resolved the problem. I pick out seams repeatedly if they aren't working out. No fraying. I would try and find where you can let something out a bit more to get the whole block to 12.5".
Last edited by sewbizgirl; 07-15-2018 at 06:11 PM.