This is a crazy way to do it, but if you put your triangles together, it will form the center. Measure that and then you could figure out the center triangles. Don't forget to add the 1/4'' all around . then you might be able to make triangles to form the center block. use some scrap fabric and make a sample. This is how I would do it. Hopefully someone will have the piece you are missing in the pattern and can tell you what to do.