hate to belittle you buuuut I think there's a simpler way to do it. Just look at Wazoo's tutorials: http://www.paulwasserman.net/SHIII/#Step%205:%20Get%20The%20Target%20Speed%20From%20Y our%20Observations%20(DS)
If you make markings every 3 minutes and 15 seconds then the distance between in the two equals the speed ie. 300meters covered in 3 min. 15 secs. would be equal to 3 knots. Read his stuff. It's cool.
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