I have a Tri Data St 60 that I calibrated with GPS under calm conditions with slack current. It allows me to see boat speed thru the water. Also the difference between the two readings while underway (GPS / Tri Data) gives me the speed of the current.
Speed through the water. Go uptide, downtide, look at the GPS each way and do the math. 8 knots+6 knots= 14 knots. 14/2= 7 knots. Not saying to troll 7 knots, just an example. Certainly sea conditions, rigging techniques, and target species all call for some variation in speed to achieve an attractive presentation of the baits.