Tom,

There are several ways to approach the answer to your question; this is my response?that doesn?t mean it?s the only response.

Choose your tippet to fit the conditions you are fishing under, each day/location/weather condition/fly pattern will be different. If the conditions do not allow your tippet to turn over correctly (for the type of cast you are making) change the tippet size. It does not depend strictly on the size of the fly pattern you are using. You may have a #16 XS Variant pattern, which will behave very differently than a #16 3XL pattern?hook gape alone is not the factor that will determine your tippet size. Then there are water conditions, dry or wet, wind or not, distance cast?there are numerous factors than will influence your tippet. My advice ? choose a brand you have confidence in, buy several spools in various diameters to suit the type of fishing you do most often, change to match the conditions you face. You don't need a formula.

Best of Luck,
John