Lone star help!

I'm going outside my comfort zone of squares and rectangles and trying a lone star. How do I determine the size of the triangles to finish the diamonds into a square? I'm trying to the no Y seams option.

I'm going outside my comfort zone of squares and rectangles and trying a lone star. How do I determine the size of the triangles to finish the diamonds into a square? I'm trying to the no Y seams option.