For me it's about repeatibility, not the actual down speed. Down speed is no more than a reference point .
If my rods are flying with my probe showing 2.3, I keep running at 2.3. I don't care what the acutal down speed is, or what my SOG is. When you change direction in the Lake O. currents, Adjust your throttle to get back to the 2.3 downspeed reguardless of SOG. I don't care if my probe speed is "calabrated" or not.... just repeat the number that's catching fish.
Just my $.02