The problems I had with Bonnie's Tulip Fields block made me think a bit more about seam allowances (and also to consult the invaluable 'The Quilters Ultimate Visual Guide' for the final verdict). I worked it out bit by bit. For any straight seam, you cut a square which is half an inch bigger than the finished block you need, because each side has a quarter inch seam and two quarters are one half. So far so good.
For a half square triangle, you still need the quarter inch seam each side, but you have a diagonal seam in the middle as well. Although that is a quarter inch seam when you sew it, because it's slanting, it takes more fabric, so you need to add three eighths of an inch to accommodate that. So, the total you have to add to the finished size for cut size is seven eighths (a quarter inch, another quarter inch and three eighths of an inch.) Still with me?
So for a quarter square triangle, you need to cut the half square triangle in half. That adds another diagonal seam, and thus another three eighths of an inch to the previous total.