need help with speed estimation
 

Can someone please outline the steps to use the target speed calculator on the TDC? It never seems to work for me. I have been doing a manual speed calculation with the nav map but it would be nice to have the crew help me out once in a while. One question I have is if I put my scope up just long enough to update the plot, (send bearing, range and aob to TDC), and , them put the scope up and to it again say five minutes later, will the TDC calculate the movement from the last date entry or to I have to take two bearing and range estimates while the scope is up in order to get the speed calculator to work?
Thanks!