Hi Chris, You're right with trying to match the flow, very difficult. Iv'e tried this a long while ago, while Czech nymphing on the Diemel in Germany, using a bright indicator fly on a dropper.
It's very hard to concentrate for long periods, on watching the dropper for current pace and drift. Which I suppose in some ways, explains why certain observant fishermen, are very good at judging water movement.
A well versed and practised fisherman, will know how vital fly placement and how current drift, can effect how a fish moves toward a fly. Any unnatural movement will send the fish elsewhere.
Thank You Chris, Nice observation.
Best Regards
Stuart