Backing in in general all depends on the kinds of fishing you will be doing. For normal trout fishing about 25 to 50 yards of backing is all that is required. For larger fish like Steel head, Salmon and of course salt water then I would goo with a least a minimum of 100 yards of backing. You really half to ask yourself, what is the backing for any way. For light fishing it allows you to attach the fly line easy to the real very easy by using a simple nail not. It aslo keeps you from damaging a DT line that way as well. With out backing, you would have to try and tye the fly line to the real which could be done, but would be a mess. Now backing is also used for fishing that are strong enough like carp to take all you fly line plus more. IF you ever fight a 15 lb Carp, you will be very thankfull that you had backing on the reel. Same hold true for steelies and so on. On my fly reels that will only see normal Trout action I use a minimum omount of backing lfrom 25 to 50 yards. I stay with this rule of thumb on a smaller large arbor real as well. I really wouldn't worry so much about those charts on the back of a fly reel box. So depending on what your fishing for would dictate the amount of backing to use. I feel less is better unless I am going after Carp, Bass, Steelies, Salmon and so on.
Andy B