It is very difficult to get an exact distance to an HVC; the best method of determining a limit on the distance to an HVC is by observing two stars, one in front of the HVC and one behind it, that are not separated by a large distance in the sky. By determining the distances to these two stars, one can then get a limit on the distance to the HVC. This, of course, depends upon the assumption that the metallicity in the given HVC changes very little between each sightline.
