Manual TDC speed figuring

[pythos]

I haqve been having a very annoying thing occur with my speed figuring when setting up a solution.

When I get a visual contact, I stay out of visual detection range, but keep track of the target's position. On the map, I place a mark at the reported position (map updates on), and start the watch. Wait 5minutes, and place another mark. Measure that distance, and work out the speed using the nomengraph.

Every time I have done this it has been wrong, usually to fast, which causes the torps to zoom ahead of the target.

Using the crew to figure the speed is worthless (500 plus knots most of the time).

anyone have better methods for figuring speed for manual TDC? Or is there a possibility that this is an simulated bug of the MK14 torp.