I think that we are getting a couple different rules that serve
different purposes confused in the course of this discussion.

Both 22.2 and 22.3 emphasize that one selects the predominant form of
usage as the basis for one's heading. In 22.2A:

Choose the name by which the person is clearly most commonly known, if
there is one. Otherwise, choose one name or form of name according to
the following order of preference:
a)   the name that appears most frequently in the person*s works
b)   the name that appears most frequently in reference sources
c)   the latest name.

22.3A goes on to discuss using the fullest form of name in use by the
author and how to select that. While the lead-in to that discussion is
that one has found *headings* based upon a usage that differs from the
usage of the item in hand, the actual discussion is based upon the
actual usage found on the bibliographic records. 

It appears from the discussion that we*re getting a little too hung
up on the 80% *rule*  The LCRI simply says to do this quickly, use
common sense, and, if all else fails, use the fullest usage found:

When there is no commonly found form * choose the fullest form as the
AACR2 form.. (When determining the fullest form for a person who uses
both forename initials and forenames, make no distinction between
initials and forenames, e.g., "B.E.F. Pagen" is fuller than "Bernard
Edward Pagen.")

As Mr. Russell has not been blessed with additional forenames and
initials as have others of us, it does appear that your heading is

100 1 $a Russell, Marc James, $d 1975-

An historical footnote: During my training in the Dark Ages, the 80%
rule derived from two sources. (1) the assumption that an author adopted
a regular usage over time and that a predominant form could eventually
be determined and (2) it kept us from revising headings in the database
until we were certain that a standard usage had evolved.

Anthony R.D. Franks
Head, Cooperative Programs Section
Cooperative and Instructional Programs Division
Library of Congress
202-707-2822 (voice)
202-252-2082 (fax)