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.
Mattress Dimensions. A Twin XL is the same width of a standard twin mattress, but adds 5-inches to the length. The overall dimensions of the mattress are 39 inches wide by 80 inches long. The additional length makes it unlikely to fit on an existing twin bed or frame.