I always centre the triangles on the centre of the other side. If you look carefully the centre of one triangle is further one way than the other. Remember the triangle should have a very small 1/4" triangle at the top where you begin to sew. Also a press without seam should also help. If after that still not right sizes either down size or make the last 4 triangles larger and centre.then cut down. At present I think the triangle is a 61/4" squAre cut in half. Try for one only a 7" square cut in half then centre in place. You may be able to make smaller after the first try. When I do this play section I use my scrap fabric.