Generally speaking, fly fishing tippet size is determined by the size of the fly you're using. Many Fly Fishers use the formula (fly size/3 = approximate tippet size)to get into the ballpark.

For steelhead (and another Northwest perspective), size #2 through #10 flies generally describes the range of flies employed with size #4 through #8 covering most situations. Size #4 through #8 flies balance and turn over best with 0X (.011") and 1X (.010") tippets. One size up or down on either end (i.e. 2X (.009") and 02X (.015")) probably gives you the high and low range for the #4 through #8 flies. Of course, nothing is written in stone and external variables (e.g. water clarity, size of fish) as well as varience in size between manufacturer's materials may influence your decision to override this logic.
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