John, you show your work beautifully and get full marks.
For those of us who want to cut to the chase, 1 mph = 1.46666666 ft/sec. or a little under 1 and a half fps.
Next math problem: If the fish ignoring Ed's fly is 21 inches long and the stream is flowing 1.3 mph, how long will the fly take to pass down the length of the offending fish?
Ed