Yes, all of the above is fine, IF you get a line...
but with convoys that are say 35km out you don't get a line, you only get that when the sonarman finally decides that the ship you've been tracking for an hour is an actual honest-to-god contact
otherwise it's just a bearing and your best guess at the distance from what you can hear yourself.
if you start working with those lines and extend them to 35km the margin of error becomes fairly large
nevertheless, listening for contacts yourself is fun and useful
