OOh, just missed the editing window...
Actually, you can use the same calculation that you use to calculate the size for setting triangles. So multiply the finished size of the square, 3 3.16 in this case (3.1875) by 1.414 which gives you 4.5". Half of that would be the FINISHED HST size you need, 2 1/4 in the example, which is what they said in the video.