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.
In the U.S., a double mattress is 54 inches wide and 75 inches long, according to the consumer website Mattress Sizes. According to the website of The Better Sleep Council, double mattresses are only big enough for one adult. Standard twin mattresses are 39 inches wide and 75 inches long, according to Mattress Sizes.