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From: "Arch" (clip) I've had it wrong. In my concept of 'offset' the
turning cone's base is at the headstock and its point is at the tailstock.
In 'eccentric', it's the reverse, but there's no essential difference.
Right?(clip)
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Arch, as far as I know, the accepted method of turning a piece at an angle
to the lathe axis uses the Escaulen chuck, or equivalent, which holds the
wood on-axis at the chuck end. I don't know of any authoritative definition
of this as "off axis" or "eccentric". As far as I know, a piece remaining
parallel to the axis, but supported off axis, could also be called "off
axis" or "eccentric"

You are not wrong--you are exploring no-mans-land.
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Leo, practically, how forgiving is your chuck re different blank lengths
while keeping the tail-points reasonably true?
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As long as some part of the wood is on axis, regardless of the distance from
the vacuum chuck, it seems to me that a live center could be brought up for
support. If this ends up being part of the turning--good. If not, you can
always turn it away just as you would a little remnant on the bottom of a
bowl. Many of these "off-axis/off center" turnings end up kind of weird, so
the decision of what to do with the end where the live center touches may be
made at the last minute.

To answer your actual question, the actual tolerance will depend on the
diameter of the piece, and the angle, since that will determine the range
over which some wood still crosses the centerline. Believe me, this becomes
clearer after you have played with it some. Failure is a good teacher, if
you can remain calm :-)