A number of factors determine tippet strength. One of the most important is the action of the fly. A small fly requires a thinner tippet than a larger fly, or it will lack lively action in the water regardless of the rod size used. Another factor is the flexibility of the rod. Years ago I tried catching some huge saltwater fish on 6-pound tippet with a 9-weight rod. In the beginning I lost almost all of them because the rod was so stiff and unforgiving with such a light tippet. If you have to prevent a snook from heading back into the mangroves, then you'll need a stronger tippet than if you were in open water. If you are fishing deep in the water column, you can increase the strength of the tippet without sacrificing strikes. There just isn't a simple formula.