The following of some of the definitions of the word "dead" and each shows why
the word is not used in the context of the guestimated, vague navigation called
Ded(uced) Reckoning:

Exact; unerring. the dead center of a target

Absolutely; altogether: You can be dead sure of my innocence.

Directly; exactly: There's a gas station dead ahead.

[In golf], a ball is said to lie dead when it lies so near the hole that the
player is certain to hole it in the next stroke.

Dead ahead (Naut.), directly ahead; -- said of a ship or any object, esp. of
the wind when blowing from that point toward which a vessel would go.

Dead Center (Mach.), either of two points in the orbit of a crank, at which
the crank and connecting rod lie a straight line. It corresponds to the end of
a stroke; as, A and B are dead centers of the crank mechanism in which the
crank C drives, or is driven by, the lever L.

Dead heat, a heat or course between two or more race horses, boats, etc., in
which they come out exactly equal, so that neither wins.

unerringly accurate; "a dead shot"; "took dead aim"

sudden and complete; "came to a dead stop"

completely and without qualification; used informally as intensifiers; "an
absolutely magnificent painting"; "a perfectly idiotic idea"; "you're perfectly
right"; "utterly miserable"; "you can be dead sure of my innocence"; "was dead
tired"; "dead right"


well, maybe those who don't know the meaning of the word "dead" might use it in
some weird fashion, but they don't count.