In both Register and NGSQ styles, biographies include a lineage or parenthetical lineage list.1 While these can be more complex and the styles have typography requirements, a basic lineage line uses names and generation numbers to summarize the relationship between an individual and earlier generations or ancestors. (The summary is placed in parenthesis, thus the nomenclature, parenthetical lineage.)
US researchers commonly assign the immigrating ancestor a generation number 12 for these lineage lists. Each successive generation (more recent) is assigned one number higher. John Fitzgerald Kennedy, Jr.'s relationship to his immigrating ancestor, Patrick Kennedy, is used as an example of a parenthetical lineage line/list, below.
John Fitzgerald5 Kennedy, Jr. (John Fitzgerald4, Joseph Patrick3, Patrick Joseph2, Patrick1)
What should I do if I haven't yet know the identity of the immigrating ancestor?
Other than in biographies, I use parenthetical lineages/lists frequently in correspondence, because it helps distinguish among the many descendants of the same name and same era.
I think the answer is to assign the number one (1) to the most ancient descendant of or immigrant. If that person is not the immigrant, then some clarification could be made. So the relationship of my great grandmother (Ellen Miller) to her most ancient Carle ancestry would be described:
Ellen Rebecca5 Miller (Rebecca Firestone4 Carle, Dirck Low[e]3, Joseph2, Ephraim1*)
*Ephraim Carle's ancestry is unknown.
1 BGC Genealogical Standards Manual, 2000, p. 101-102
2 Joan F. Curran, Madilyn C. Crane,and John H. Wray, edited by Elizabeth Shown Mills, Numbering Your Genealogy: Basic Systems, Complex Families, and International Kin, rev. ed. (Washington: NGS,2008).