Look at this page from quilter's cache - it gives some VERY HELPFUL information on how to construct the triangle in a square unit.
http://www.quilterscache.com/F/Fifty...ock_Page2.html
I think the easiest way to get a certain unit size is to draft the square - draw the triangle in it - cut it apart - add 1/4 inch seam allowances to the pieces.
Sewing two rectangles on the diagonal (like for HSTs) - and cutting down the center - will result in kite shaped pieces.