For me, when unsure of yardage requirements I work in square inch increments. X number of pieces at Y size = Z number of sq. in. multiplied by the number of pieces/blocks. Tedious but it works. Plus I add anywhere from 1/4 to 1 yard extra depending on number of cuts or how large I need the pieces to be (i.e. backing). I normally don't buy backing material until I have the top at least nearly or completely pieced in case there are changes along the way. I usually overbuy by 1-2 yards so I can maneuver the pieces to get the most efficient use of fabric. That's where my (meager) stash comes from.