When you join binding strips with a straight seam, you end up with 12 layers of fabric (in addition to the bulk of your quilt) at every join. This makes for a lumpy binding. When you join with a diagonal it spreads this bulk over a larger area so that it doesn't make a lump. That said -- If you use 3" binding you will lose 3" with every joining and will never get enough binding out of one FQ. You would be better off buying a half yard (6 strips) or 5/8 yard (7 strips). This will make for less joinings. After you cut off the selvedges and join the strips together you would end up with maybe 230" of binding (6 strips). Also, I have never made binding to end up at 1/2", but in order to get good mitered corners with that width, I think you would have to sew the binding on with maybe a 3/8" - 1/2" seam instead of 1/4". Good luck
)