If you are using 15 lb weights your probe is probably running true. Altho with blow back it is probably only running 70 ft instead of 80. (Strictly a guess). Having to run 4.7 to maintain 2.1 is pretty common. I've actually had to almost max out my 9.9 kicker to maintain 2.5. There are just too many variables to lock this down to one scenario. Current, waves and wind throw too many curveballs. Adjust your speed frequently and keep an eye on your downspeed. When u catch a fish. Note what the down speed was and then duplicate that.