ahhhhh I see .... you were meaning "just" the triangles, and not the squared up HSTs!
So I think you are trying to do a Square in a Square unit, right?
I just happened to pull this out of my sleeve for you!!!
And I too would oversize ... I always find that those things go a little wonky, no matter how perfectly you do them! So oversize, and starch ... and trim to squared up perfection!!!!