To be really accurate you should start your watch at the UZO or periscope, go the map screen make sure the repeater is in view and working and then wait until the second hand passes a nice even number like 10 sec. Make your mark and then wait 3 minutes and 15 seconds which will show up on the stop watch as 3:15 plus in this case 10 extra seconds (3:25) and mark again.
I've been using the 3:15 rule for quite some time and jump between screens to set the watch, mark position and so on. There are slight timing/postioning errors when doing this but it still is accurate enough to get the speed within a knot
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