My speedo reads in hundredths of a knot
. - K.XX and when conditions are stable you can really see how various trim changes effect speed. The speedo need not be perfectly accurate for this use, but the trends are still apparent.
I tow a 10' Alum bottom RIB
with a towing bridal which leads to port and stbd cleat so I can position the tow almost anywhere aft of the boat
For motoring I tow the dink on top of the first following wave with the crest just forward of the center of the dink. You want the tension on the line to be as little as possible (obviously) indicating the least drag.
For sailing I tow it down to leeward and as on the crest of the first following wave. If the waves are large and long then I move it forward so it is on the face of the wave. It shouldn't surf down and hit the transom or fall back an snap the tow line.
I've had no luck with super long tow lines.
In real big chop the stern is riding up and down and so is the dink, but usually there is little you can do to mitigate this.