I guess by trial, error and experience.
It's the same when you plot the target's true course and speed on the Nav.map with radar.
By the time you move to the A-scope from the PPI screen, read the distance and look up to the bearing dial, go back to the Nav. Map and select the appropriate tool (triangle thingey
), about 10sec. will have passed (if you selected the correct tool otherwise 12sec. will have passed)... Where to put the first marker?!?
Put it where the sub is 'now' and draw the line at xxx degree bearing will put you +-150yrds off... Not a big deal IF you do it the same way each time, mess up once and you'll be off by a couple of degrees and that will mess up your plot.