Even if you find it, it most likely will not be of the same dye lot. If you haven't already used it, suggest you return and find something else. Another alternative is to have one painted wall.

True.

There's a Android app called Handyman Calculator that has calculators for all sorts of stuff, including wallpaper and cement, as well as converting the standard imperial lengths and weights to metric and other.

You can also try putting paint on the lower part of the wall, a chair rail and having the wallpaper above. In most places you don't notice the lower half of the wall anyway. Or if you are missing just a bit, put a border or increase crown moldings and baseboards.