Sampioen, actually it's a very good question. In most instances, the "zone" for the drag free drift is equal to a trout's cone of vision. The closer to the surface that a trout is holding, the shorter the zone for a drag free drift becomes. There are a couple of general situations that you can be dealing with.

Situation 1; you see a trout actively rising and taking either duns or emergers. Cast 2 to 3 feet above the rising fish, mend if necessary and maintain the drift until the fly has passed over the fish.

Situation 2; you see a likely holding spot but no active fish. Again, cast about 3 feet above the spot, mend if necessary and maintain the drift until the fly has passed through the likely holding spot.

There are times when I'm just searching areas where I'll try and get the longest drag free drift I can. This may require me to feed additional line on to the water as the fly moves below my position. This is accomplished by feeding line with my line hand as I move the rod tip left and right, parallel to the water to produce S patterns on the water. The danger here is that you will miss a strike because the fish will take and reject the fly quicker than you can set the hook.

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Dan S
Slow down and savor the moment; it will be over with all too quickly