OK, here is the calculation....
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 finishes size x 1.414 /2 + 7/8 cut in half on the diagonald
Your example:
Since your corners span two blocks, the finished size is 24 inches.
24 x 1.414 = 33.936 round to 34, divide by 2 = 17 & 7/8 = 17 7/8. Cut in half on the diagonal giving two half sqaure triangles.
I went to my cutting mat and measured.
You can follow this too and see that it is what you want.
Lets start with the corners.
The corner triangle spans TWO 12 inch blocks and you are going to want to make it from a half square triangle because you want the bias edge to be against your blocks and the straight of grain on the outside two sides.
So, the finished length of this seam across two 12 inch blocks is 24 inches.
Take your 24 inch ruler and go to your cutting mat and lay it diagonally across the corner at the 17 inch mark you see the size of the finished triangle is 17 x 17 x 24. So this is the size your triangle will be after sewing into your quilt. Now, you need a 1/4 seam allowance on the 17 inch side and 3/8 inch on the diagonal or 24 inch seam and another 1/4 inch for the other triangle. So, thats 7/8 inches added to the 17 inch measurement (17 7/8 inches) to get the size of the square. Cut this in half on the diagonal and that will give you your corner triangles.
What do you think?
Judy