Target speed can be calculated by pointing your sub's bow just ahead of the target with your periscope at 000. You start your stopwatch the moment the target's bow crosses the periscope hairline and stop the stopwatch the moment the target stern crosses the hairline. You have now measured how long it takes the target to travel it's own length so you have the target speed in meters per second. We want the speed in knots so we have to convert m/s to knots.
Converting m/s to knots is simple: to go from meters to nautical miles you divide by 1852 and to go from seconds to hours you multiply by 60x60 (seconds ---> minutes ---> hours). So the total conversion factor becomes 3600/1852 which equals 1.944.
To use the slide rule to calculate speed put the hairline on 1.944 on the D scale (on this particular slide rule you can also use the 7 on the outer scale which might be easier to remember). You now match the number of seconds you measured on the D scale with the target's length on the C scale. The target speed in knots can now be read directly on the C scale at the hairline.
In this example (see photo) a large steamer with a length of 141 meters took 36 seconds to travel it's own length which gives a speed of 7.6 knots.