This is a tutorial on how to make 8 half square triangles (HST) at a time. Whats nice with this method vs the 4 at a time is that on this, the bias is where its supposed to be and your final edges are on the straight of grain (since that has messed me up with the 4 at a time method where you sew on the straight of grain and end up with bias edges).
Math: I started with 5 squares and my HSTs were each slightly over 2 (They will ultimately finish at 1.5 in the quilt). You will lose about 1 with this method. So if you want HSTs to ultimately be 4 finished in the quilt, they would be 4.5 unfinished. You would want to start with 4.5 x 2 + 1 = 10 square. If you want 6.5 finished, 7 unfinished, you would start with 7 x 2 + 1 = 15 square.
Let me know if you have questions
Put 2 squares right side together. Draw 2 diagonal lines on one
Sew 1/4 on either side of each line (4 seams)
Using the center as a guide, cut a horizontal and vertical line