Just now I have started a "Hole in the Barn Door" quilt, that calls for half square triangles. I noticed that it calls for the squares to be 4 7/8 in order to match up with a 4 1/2 piece next to it. This would mean add 3/8 inch to the square that will be cut to a triangle. I am pretty sure that this would hold true for any size, but just to be sure, let some of the more experienced than me, back this up before you start cutting. :-)