Vancouver is a city on the north bank of the Columbia River in the U.S. state of Washington and the county seat of Clark County.
The width of the twin quilt is 66 +12 (for the two extra columns of 9 patches) + 4 (for the two extra strips of sashing) =82, which is a bit wider than your target of 78, but close enough. Plus, it's nice to have it a bit too large than a bit too small.
If you were making a twin quilt, you would be just a little long for a comforter size, which would be perfect. If you are trying for a queen size comforter, you want about 86 X 88 inches. Divide 86 by 12, and you get seven, and 88 by 12 is also seven.