Kim
Are you running NMEA0183 on your system and does it have GPS data on it. If it does as mine does, i just wait until slack tide and set the speed through the water STW (data from the log paddle wheel) to match speed over ground SOG (derived from GPS data). I get the boat up to 5 or 6 knots constant speed SOG (GPS) and then go into the tridata instrument and set the STW to match the SOG, works every time. I adjust mine 2-3 times a year as the accuracy of the paddle wheel varies dependant on when her bottom is cleaned and antifouled
If you do have GPS data on your 0183 bus i can probably find the Raymarine instructions of what to do, it takes 30 seconds
We ended up doing it this way as trying to do it the full way over a set time/distance at slack tide in the Solent is way on impossible, there is almost always a tidal affect
John