Well on half-square triangles the rule is, take your finished size and add 7/8" to it.
If you want a finished 12" block using only 4 half-square triangle block each block would be 6" finished - so the squares that you start with would be 6-7/8" then cut diagonally. A lot of people overcut these squares, cut diagonally then after the 2 triangles are pieced together and pressed cut them down to the correct size (in this case 6-1/2" unfinished).
You may want to wait for another answer to validate this!!!!