Hi,
We have been thoroughly confused by the syntax of the formal public identifier (FPI) as well.   There are a couple of examples on the RLG's webpage, under "Site Configuration Guidelines for Finding Aid Contributors," http://www.rlg.org/rlgead/siteconfig.html
There's also sone directions (which in part contradict RLG's) at the berkeley site for retrospective conversion guidelines:
http://sunsite.berkeley.edu/amher/upguide.html

Currently, the <eadid> we have for a finding aid have called "Dell" is:

<eadid publicid="-//US::Newberry Library::Roger and Julie Baskes Department of Special Collections//TEXT US::ICN::Midwest MS Dell::Dell.sgm//EN" countrycode="us" mainagencycode="icn">Midwest MS Dell</eadid>

We're in contact with RLG and our outside consultant to make sure this is phrased correctly, because, as you said, it's difficult to find examples of the syntax.  I'm hoping my example will spark some discussion on the topic, and thanks for bringing it up.

************************************************
Alison Hinderliter -    [log in to unmask]
Project Archivist
The Newberry Library
60 W. Walton St.
Chicago, IL  60610-7324
(312) 255-3694