I don't know where the previous post is, but maybe this will help you.
Take your finished size of 1/2 square triangles (let say 6" square just to have a number to work with). Add 7/8 in to this for 6 7/8". Cut your two fabrics at 6 7/8". Lay them on top of each other right sides together. Draw a diagonal line between opposite corners. Sew 1/4 inch on both sides of the line. Cut on the line and you should have 2 squares that are 6 1/2" inches.
If you try this one time and you're not getting 6 1/2 inches but are really close, you can cut the squares at 7" and trim excess.
There is a way to do this with LARGE pieces of fabric. You will need to draw a grid using the measurements from the steps above, draw your diagonals in one direction, sew, then cut on all drawn lines. Remember, every square in the grid yields 2 half-square triangle units. Keep that in mind when figuring how many to do.
If I could reply with a pic, I would. Hope I didn't completely confuse you.
Don't know about the flying geese method.... sorry....