font sizes in ems

In a response to another poster (and a slightly different question)
David Perrell wrote:

"I believe the method most likely to give a reader legible type is to
use only em or percentage values because these numbers will be relative
to the reader's preferred size. ... For example, you might want to
specify font-size for P at 1.2em and font-size for H1 at 2em. Your
paragraph text will then be 1.2 times as large as the reader's preferred
default for BODY and your H1 headings will be 1.7 times as large as your
paragraph text."

I've seen this opinion expressed several times on this list. However, my
experiments with both NS4 and IE4(p2) show that the example does not
work as given, and indeed it shouldn't. An em (which is inadequately
defined in CSS1) is, as best I can understand, the total size of the
font, measured more or less from the bottom of the descenders to the top
of the ascenders, or roughly the width of a capital M in the font.
(Historically this gets very complicated, but let's start from there.) 

If I specify a font size of 1em, then, it comes out considerably larger
than the default body type size of the user. I've experimented with
several different default fonts and sizes and find that in order to
approximate the user's default size you need to specify something
between .6 and .8 em, with .8em being the most reliable conversion among
those I tested. This, of course, varies with the font... and to a
certain extent with the browser. For those of you who are interested,
I've posted a demo with an assortment of common Windows fonts at:

Therefore, I can't see how one can advocate starting with an em as the
recommended unit for an initial font spec, since users' default fonts
are liable to vary considerably. Or can one recommend a standard
conversion value for an em to user's default, say .8?

Another typographer friend of mine has this to say:
 "[Designating font sizes in ems] is literally nonsensical in
traditional terms.  An em is a unit of space; 12pt is a unit of size ...
it is meaningful to say "a 12-pt em" or "a 12-pt font" but not a "1-em
font" -- the size of the em varies according the size of the font, not
vice versa."

Yet I seem to see designating font sizes in ems being advocated in this
group. Do I misunderstand? Any clarification gladly solicited.

Gayle Kidder