IS20 / DST800 speed calibration for NMEA0183 devices?

Hi,
As ths is my first post here, first many THANKS to all of you for the wealth of information found on this forum!

My sailing boat has a primary Simrad IS20 instrument system with an Airmar DST800 triducer, and an AT10 converter feeding NMEA0183 data to the (non Simrad) mast repeaters and a PC tactical racing software. My problem is that the boat speed value sent from Simnet to the NMEA0183 devices (using the AT10, xxVHW sentence) appears to be the uncalibrated value from the sensor instead of the calibrated value displayed on the IS20 instruments. The difference corresponds to the 1.15 calibration factor which I have entered in the IS20 Graphics instrument. This affects the values shown on the NMEA0183 speed and wind repeater on the mast, as well as my PC racing tactical software. As the speed repeater has no local calibration possibility, I am at the moment stuck with this discrepancy in speed values.

My question is, have you noticed this discrepancy and is it dependent on how you calibrate the IS20? What I mean is that the speed calibration can be done in two ways: either by entering a manual correction factor, as I have done, or by using the "Calibrate w/SOG" function, whereby the correction factor resets to 1.00.

The text (from the user manual) appears to suggest that if I manually enter a correction factor, it is only stored in the IS20 instruments. Whereas if I use the "calibrate w/SOG" option, the calibration data would be stored in the DST800 sensor itself and then the calibrated speed would also show up on the NMEA0183 output.

This would be what I need. But I'm a little hesitant to try it out because then I lose my current 1.15 reference value, so if any of you can give advice I'd appreciate it. The speed sensor is an Airmar DST800.

My second question is if there is a calibration tool which I could use to calibrate the DST800 speed sensor directly? My brief search here revealed the Maretron N2KAnalyzer, but that tool evidently requires a fairly expensive USB100 N2K-USB gateway. Can the N2KAnalyzer do the sensor calibration- and possibly even including the more fancy linearization features described in the DST800 manual (PGNs) ?

I am a little hesitant to invest in the USB100, especially as I already have an Actisense NGW-1 in the system which has served my needs so far for connecting to AIS and plotter. Any alternatives?

Third, if all above fails, I will need an NMEA0183 box that can apply a programmable conversion factor to the xxVHW speed value. If you know of one, I'd appreciate to know. I have looked at the Brookhouse multiplexer which has many programmable functions, but unfortunately it can not do the "multiply by correction factor" operation.

Thankful for assistance,
Nick