calibration of i40 bidata speed

[georgeo]

I have a Tartan 34C sailboat with i40 Bidata and B744V transducer. Over several sailing seasons I'm trying to improve the accuracy of the speed data. I understand the difference between GPS speed over ground and the speed through water that the transducer measures. In Lake Superior we really don't have currents that would make any significant difference.
My theoretical hull speed is 6.7 knots.

After collecting numerous data points comparing an independent GPS speed with my i40 speed display I am finding at higher speeds (4-9 knots) the i40 reads maybe 15 to 25% high while at lower speeds(0-3 knots) the i40 reads maybe 10% too low. At about 3 knots they seem to agree.

So I was prepared to do the calibration described on page 34 of the user manual, but when thinking about the calibration factor I now realize I don't understand what this is. Is it a pure factor applied to the transducer sensor speed signal? If I set it at 0.80 this would normalize at high speeds with GPS, but then read too low at low speeds, where the i40 is already low.

What specifically is the calibration factor? Is it a linear factor to the sensor data?

What specifically is on page 33, item 5 the speed response level ? And what are the implications of its setting?

I'm hesitating to make any changes to the unit until I understand what these are.

Thanks for the help!

George