Another method of doing this quilt would be to make flying geese units. You eliminate the center seam that way. But you asked about doing HST. I would do the grid method described here:
http://www.patchwork-and-quilting.co...triangles.html
If each angel is to be a different color mark your grid on the light fabric 2 x 1 (2 squares side by side). So cut a rectangle of each fabric, light and dark a little over sized (if I were doing it I would cut 7 1/2 by 14 1/2" rectangles) and draw two side by side squares that measure 7" square. Draw your diagonal line through each square and sew a 1/4" on both sides of the diagnal line. Cut apart on all your drawn lines and you will have 4 HST that you can trim down to 6.5". Then sew together your angel blocks.