Sorry, I clicked Send before fixing up my typos in the Chipman
quotation. Here's how it's really written:

"Fortunately, the combination of circumstances that would require
accurate information about th proximity effect factor for distributed
internal inductance occurs rather rarely in transmission line practice.
The most unfavorable situation would be a parallel wire line with solid
circular conductors, the facing surfaces of the conductors being
separated by only a few percent of a conductor radius, operating at a
frequency to have a/[delta] have a value near 2. These conditions make
the distributed internal inductance comparable in magnitude to the
distributed external inductance, with a proximity factor that might be
as small as 0.8 or 0.85. There is no recognized basis for making an
accurate analysis of the total distributed inductance of a line for such
a case.

.. . . When the facing conductor surfaces are at least a conductor
diameter apart (s/2a = 2), the distributed internal inductance will be
less than 20% of the total distributed inductance, and the proximity
effect factor will me not less that 0.87. . . Proximity effect can then
not modify the total distributed inductance value by more than about 2%,
and the factor need be known only very roughly. . ."

(*)And even this is a high-frequency approximation which assumes that
the conductors are at least several skin depths thick. Expressions for
line inductance without this assumption involve Bessel functions, which
I assume would also appear in expressions for Z0.

Roy Lewallen, W7EL