“There are two easy formulas that will help you remember tippet sizes, diameters, and what size fly to choose. The diameter in thousandths of an inch is always 11 minus the X size, so a 6X tippet is 11-6, or .005”. And to get the approximate tippet size for a fly, divide the fly size by 3. By this formula, a size-12 fly takes a 4X tippet, and a size-14 fly is a 5X tippet. Just don’t forget that in clear water, you might want to go to one size tippet smaller, and in dirty water, you might go to one size heavier.”