Can it be assumed that any off-cuts of parchment prior to it being ruled and cut to size (i.e. to write a sefer Torah etc), has been made with a tnai and can be used for "anything" as long as its not tashmish gnaiy? For example, can i use it to a make artworks? Can I practice writing on it prior to write a mezuzah etc? Can I have bar mitzvah boys (as part of a class) try writing on it?