You can also use easements by combining the various curve size. I combine O-48 and O-60 to get an "O-54" curve; it isn't true O-54 but it has O-60 easements and it tightens up the spacing between the loops. I've done this with O-36 and O-48 as well to create an "O-42" loop. I would assume you could mix O-72 easements with O-60 curves and wind up with an O-66 loop as well but I haven't done it. At the moment I have an O-60, an "O-54" and an "O-42" around the tree. Any more than that and it just gets too messy for me.
Just a thought...