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 most commonly offered blanket and mattress sizes are: Twin-size blanket, about 66 inches wide by 90 inches long; fits twin-size mattress, 39 inches wide by 75 inches long. Twin XL size blanket, about 66 inches wide by 95 inches long; fits twin XL size mattress, 39 inches wide by 80 inches long.