I must admit that I generally choose my tippet size based on:
1. Water type; i.e., is it gin clear and smooth water - then a finer tippet.


2. Average fish size; i.e., if the fish are generally below 14 inches - then the lighter leader

I do this instead of relying primarily on the size of the fly. Obviously, when you go to EXTREMES ( big streamer or tiny #20 dry), there must be adjustments. But, I generally choose tippet size based on water type and the expected size of fish in the particular water.