For the setting triangles, take the finished size of the block x 1.414, round to the nearest 1/8 and add 1.25. Cut your square that size and cut on the diagonal both directions giving you four quarter square triangles
Your example:
12 x 1.414 = 16.968, round to 17 + 1.25. cut your square 18.25 and cut on the diagonal both directions.
For the corner triangle:
take the finished size x 1.414 /2 + 7/8 cut in half on the diagonal