Bava Batra 162: Empty Space

More on documents and the implications of establishing all kinds of things on the basis of a document's authority. Including an important regulation that the last line of such a document cannot have new information, so nobody can mess with its content to add new details there. Also, what about a line and a half is left blank? When 2 lines and more is invalid and 1 line is valid... does 1.5 lines follow 2 or 1?