Maybe I missed the intent of the thread?
Are you talking about what folks refer to as "an Attractor" or "a Searching Pattern"? Meaning there are no consistent rising trout and you prefer to fish a dry as opposed to a wet or nymph pattern.
If not, you would presumably be fishing a hatch. Most "decent" representations of the hatching insect will work, given good presentation.
If fishing below the surface, screening the water or turning rocks over will usually show you what a fly that will work should look like for your selection.
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