When you center the triangle section to the center section you can match up the edges and then fold back the fabric til you can see where the white connects between both on the seams. this is what you want. If it is not matching then you need to move it over til it is matching. This will give you points that match after it is sewn.
The second picture here shows where I didn't get it to match and I had to take that out and redo it so it matched right.