For large models, I always start out with 69 to 70 feet, handle to centerline, and work down from there. It's easier and cheaper to shorten lines than to make new ones, unless you happen to have several sets of spares in different lengths. Any .60 size model should be able to at least fly in circles, climb and dive on this length, and as you get the model sorted out and get a feel for things you can refine it from there. I always go by the handle to center line dimension also, just for simplicity, and that is what the line length rule is all about anyway. Get your line length figured out and comfortable, then make a spare set right away just in case!
Good luck and have fun,
Dan McEntee