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.
A typical bunk bed for a child's room features two twin mattresses. A twin mattress is 39 inches wide and 75 inches long. But a different type of twin mattress exist: a twin XL. The extra long variety has the same width, but an extra five inches of length at 80 inches long. The extra long twin has the same length as a queen or king mattress.