I calculate 10 3 inch wide stripes joined together should make a very long pieces about 205 inches which will be enough as you need extra to turn corners and to finish off. I did this with straight seams I have no idea of the way with correct diagonal.

Would it be easier as you have not done a binding before to use a gripes fabric with the colours that match. You would need to be sure 18inches. Cut across fabric so not many joins.

2" folded in half = 1", stitched to one side w/ 1/4" seam allowance leaves 3/4", folded over the back (taking into account the 1/4" on the front, 1/4" on the back- you will have a 1/4" seam allowance on each side ...you need at least 1/2" more (or 2 1/2" strips) in order to get a 1/2" on each side. ... that seems awful wide for such a small quilt- but some people do seem to like the look of wide bindings (personal choice I guess) but do understand it will be much narrower with your current measurements

What is a gripes fabric? I could not locate a definition. Thanks

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 :o)