Danger space is the region behind the ship where the projectile hits the water but only after passing through the ship first (which means a direct hit on the ship, in other words). Usually the superstructure is ignored as a first approximation and the ship weather deck average height (H) and amidships width (W) is used (assuming broadside-on fire; use ship length (L) for W when studying raking fire) . Assuming that the angle of fall is F degrees, the width of the danger space in feet or meters or whatever is being used for the ship dimensions is:
D.S. = W + H*COTANGENT(F) = W + H/TANGENT(F)
If the projectile is falling straight down (F = 90 degrees), then D.S. is merely W, while if the gun is firing at point-blank range horizontally, F = 0 degrees (approximately), and you simply can't miss as long as you have the bearing correct (D.S. is infinite).
If you want to, you can get more detailed and assume a roughly flattened pyramidal superstructure & turret area above the deck amidships and add its average height to H for a somewhat better estimate of the chance of hitting SOMETHING on the ship, though superstructure hits are iffy since much of the area is not important (excepting the turrets, secondary and AA gun mounts, radars, bridge, directors/range-finders, conning towers, etc., of course) and few will result in major damage to the ship itself, barring a turret explosion that reaches a magazine (as with the WWI British battle-cruisers).
Note that end-on (raking) fire has more chance of doing major damage as even a high superstructure hit can pass downward through that region the ship and eventually hit a turret or barbette on the far end of the ship with possibly major consequences, while a low superstructure hit will usually hit the near-end turrets or barbettes first, again with possibly major consequences.
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