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.
For the 9 patch quilt: The finished quilt measures 66 x 90 for the twin size. Each 9 patch block finishes at 6 and each sashing strip finishes at 2. Since you want the quilt to be at least 78 wide, you will need 12 extra inches. You would need 20 extra blocks (ten for each additional column) and then add in the sashing 2 finished x 2 strips.