I would add an inch or 2 and then just cut the whole square in half.
So let's say the blue square in the quilt is 6" finished, 6.5" unfinished. For the corner pieces, I'd cut it 8", cut it in half (so it can be used for 2), and trim to size. Nothing mathematical about it :) And then of course, try it for 1, see if it works before cutting all of them.
Or come on here and say "here's the size of the middle block - tell me size to cut the side blocks"