Calibrate the log

The easiest way to calibrate a log is to wait until you’re operating inside a lock somewhere – in a Dutch canal for instance – note the speed over ground (SOG) from the GPS and set the log speed accordingly. You won’t be far out. Failing a lock, you can use the same simple system if you can find some genuinely slack water as noted by the local lobster pot buoys, but don’t expect the world. Slack water isn’t that reliable.
Alternatively, choose a steady current and motor at a constant speed straight into it. You can check whether you’re achieving this by making sure the course over the ground (COG) lines up with the compass. Note the SOG, turn through 180 degrees and steam down the reciprocal at the same speed. Note the SOG again and take the average of the two. That’s your boat speed. Set the log to it and you’re done.