The mechanics of depth charge attack are not always laid out.
USN d/c have settings for Shallow, Mid-water, and Deep. Round numbers, given that these are pressure-sensing fuses, that's 50, 100, and 150 feet.
Which comes off as very shallow depths for lighting off quarter-ton explosions (1/6 ton for the streamlined Mk 11of only 350#).
But, part of that is how long it takes things to sink in seawater. And, just how deep you can keep, and maintain, a sonar contact (which is lost pretty much as soon as you ring down for either Full or Flank speed, from how much noise the plant and the screws make). Another factor is that the "effective range" decreases as a cube function for every fathom of depth. By about 200 feet of water, the d/c will need to be almost in direct contact to have any effect.
An early war "pattern" was an elliptical "horseshoe" of 8 or 10 d/c about 60-75 yards wide and 100-150 yards long (about twice the size of a sub or so).
Now, probably not a good idea to lurk around with a cople of tons of explosives going off within 150 feet of water depth of your own hull. Hence the need to put the spurs to the ship and get to full gallop before firign the barrage (the preferred term-of-art).
Actually, the stern racks were easiets for "exploratory" shots while also being most dangerous. The latter because any uh-oh in the fuzing is detonating a quarter ton of explosive just under your own counter, where the ship's structure must needs be a bit lighter than the rest.
But, the fantail also allows for stowage of a lot more d/c than by the K guns, whic honly have room for 6-8 d/c per launcher. And a stern rack will easily hold a dozen or more, often with a stowage rack adjacent.
It takes about 15 to 30 minutes for everything to quiet down to start using sonar or hydrophones. A submaring running on batteries can be very quiet.
When prosecuting an attack, one often starts with a blank sheet of plotting paper. You put an "x" down on the suspected location of the target. This will have a dashed line on the presumed directional vector of the target. You then chart out an intercept course to the point one expect the target to be by the time you reach it.
Which is complicated as you start your attack run at Dead Slow to retain the sonar or hydrophone contact. Also to detect if the target detects your approach, which affects where that dashed line goes from the "x." It was considered to be good form to make an arced attack as you dow not want to provide a simple torpedo solution for the submarine.
Oh, and your turning radius becomes smaller at speed, so you need to account for that in plotting your own course, especially as that affects where to look for your target. Which also allows you some level of prediction, as if the target is not where it is supposed to be, you can then postulate on possible locations, knowing the limitations of the target. But, you are still guessing.
This is one of the reasons for which the Hedghog was considered to be far more successful. The bombs were cheap and plentful and easier to reload. They had significant range, too. They could be launched at lower speed meaning not revealing your attack run to the target. Also, as they were contact fuzed, any "bang" you heard was likely a successful hit on the sub. If not as sure a "killing blow" as with an under-the-keel d/c detonation.
This is why the DE and DD/DDE post war saw extensive use of 18" diameter homing torpedos rather than d/c. And a 10 or 12 mile stand-off range has its adavatages, too.