Ok, that just made my head hurt!
Wouldn't the inside triangles be....ok. lets see if I can explain MY thinking on this.
Take a square, the same size at the tulips, then make a triangle out of it. That SHOULD be the size of what you need.
But, that's my thinking.
OR, this is what I'd do too....not sure anyone would agree with me...
I'd take a BIGGER triangle...sew it to the corner, then square it off. Then measure it, including the 1/4 in seam allowance that that would be the size you would need.
I'm probably way off on this, but in my head, it would work for me.
Does it make any sense???