Re: Backing Strength?
Unless you have an older reel with very small capacity 30# would be a better choice. It will be stronger than your tippet. There are darn few fish that the difference in capacity could be an issue. Fish offshore around oil rigs and a fish can snag you with your fly line out. Be a bum scene if you loose your fly line,
You can go to gel spun/braid. Even here I'd suggest the 50# as its smaller than most 20 and knot tying is easier with larger dia.
We all wish fish ran so far that line capacity would be an issue but with today's reels it really isn't. I do suggest you put enough backing that with fly line on it is full full as this slightly increases your line intake for each revolution, hey why not fill it up since we are all optimists.