So you do not show the connections/bindings and you do not model the atoms as spheres. That way they won't scale and you would have to manually scale the size down with increasing numer of cells.
While it should be possible to write routines which create the necessary vectors (at least for the orthogonal case, I don't think that Mathcad is the right tool for this task.
BTW, what your sheet shows is not the full unit cell, right? At least not according to the pictures you added in your first post.