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.
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.