You just draw your course line 4600 yards from the target track.
You do not have to be 4600 yards away, you can be any were along the your course line (500+ yards) and you just shoot at the angle of the constant bearing line.
If your constant bearing angle is say 10° just point your periscope at 10° Left or right of 0° depending on which direction target is coming from and as the ship crosses the wire you fire.
The instructions you refer to are to clear any old data from the TDC so that the torpedo shoots straight out the front. To do this you must set the periscope to 0° and pull the range all the way down to about 1200 yards. Click the send range button twice to set the gyro angle to 0°
To shoot stern tubes you do the same thing only point the periscope to 180° and sent 1200 yd to the PK.
You must also set speed and AoB to 0 for the same reasons.
If you have good accurate speed and course data this is the most acute and versatile shooting method there is, as far as I'm concerned, and Spectator's tutorial is EXCELLENT.
Magic