The pinging three times thing is only to give you the human an average reading, as the sonar gives you fairly but not very accurate readings, so taking three readings allows you to get an average reading. I don't think the TDC gets a more accurate range if you ping and send three times, or even ping three times and send once. In fact, it may make the auto speed thing record ranges that are jumping about and that may make your speed estimate worse.
I think as well that the stop watch auto speed calculater icon knows at what times you sent the readings, so you don't need to do it at exactly 3 minutes. The 3 minutes thing for the speed is for you to work out the speed in knots when plotting in yards - I think it's 3 minutes and 15 seconds or something for meters, haven't really looked at it in great detail. But anyway, the auto speed button only needs a few readings over a period, I dont think it needs precisely timed readings, so you should be able to plot and send to TDC independently. Try it and see what happens.
Edit: OK, I'm getting confused here. Are you trying to plot only by manual plotting, if so why are you sending stuff to the TDC? :hmm:
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