Rigging a DickNite and leader length is not as critical as the "how" it's being fished.

I would contend that if you are sweeping across a riffle/run with a DN, those fish will more than likely be flossed.

You can tell when you are flossing (on purpose or not) fish with DN's, as the fish will be hooked in the corner of the mouth (in or out).

OTOH.....

Park your boat above holding fish in a riffle, cast your DN past and downstream of them, and retrieve the DN back UP river. As the DN goes up and past their face, they will snap at it, if they are biters. Those fish will be hooked in the tip of the SNOUT.

Or, run a DN behind a diver. Back the DN in to holding fish in a riffle. Again, if biters, hooked in the SNOUT.

Or, cast a DN in a froggy slackwater backeddy. Retrieve back to you in a slow manner. The snappy fish will hit the DN and all will be hooked in the SNOUT. If hooked in the corner of the mouth, you probably incidently flossed it as the fish were swirling around in circles in the eddy.