A few of us "got into it" regarding backing last month (i.e. gel-spun vs. small-diameter dacron); regardless of what you choose, for an 8-wt. outfit I try to have around 200-yds. of 30-lb. backing when stalking on the flats. Granted, I've never hooked into anything that peeled off more than about 100-yds. of backing, but it's nice to know that if I need it, it's on the reel. Then again, the largest fish I've hooked on an 8-wt. was probably no more than 6-lbs.
As an aside, one thing I try and do and is purchase two colors of backing, and load half the backing capacity of each onto the spool; for example, on a spool that held 200-yds. of backing, I'd load 100-yds. of chartruese, yellow, or orange backing first, then load 100-yds. of white backing on top of that, followed by the fly line. In this manner, I know when to start worrying if the fish gets deep into the first layer of backing. FWIW...