Has anyone on the list ever worked with or for Cessna who might know
how they generate their performance charts? Experimental measurement
-- or calculated "guess"?

To answer my own question: in the USA FAR part 23 describes in
excruciating detail how these data charts have to be created and I
excerpt for GA (a bit wily nily):

Sec. 23.45

General.

(a) Unless otherwise prescribed, the performance requirements of this
part must be met for--
(1) Still air and standard atmosphere; ...

(b) Performance data must be determined over not less than the
following ranges of conditions--
(1) Airport altitudes from sea level to 10,000 feet; and
(2) For reciprocating engine-powered airplanes of 6,000 pounds, or
less, maximum weight, temperature from standard to 30° C above
standard; ...

(f) Unless otherwise prescribed, in determining the takeoff and
landing distances, changes in the airplane's configuration, speed, and
power must be made in accordance with procedures established by the
applicant for operation in service. These procedures must be able to
be executed consistently by pilots of average skill in atmospheric
conditions reasonably expected to be encountered in service.

(g) The following, as applicable, must be determined on a smooth, dry,
hard-surfaced runway--
(1) Takeoff distance of Sec. 23.53(b);
(2) Accelerate-stop distance of Sec. 23.55;
(3) Takeoff distance and takeoff run of Sec. 23.59; and
(4) Landing distance of Sec. 23.75.
NOTE: The effect on these distances of operation on other types of
surfaces (for example, grass, gravel) when dry, may be determined or
derived and these surfaces listed in the Airplane Flight Manual in
accordance with Sec. 23.1583(p).

Note the word "determined", not "calculated" or "derived" for all
except the bit about types of surfaces, where "derivation" is allowed.

There are a LOT of variables in those rules that don't lend themselves
to mathematical expressions. My conclusion is that there is no simple
formula available to apply in an Excel spreadsheet that will reliably
predict the numbers from the chart, thus your calculations seem to
have contradictory results (eg, different performance for same density
altitude).

Of course, someone with more experience in aircraft certification /
performance data generation will probably post something right away
showing I don't know a damned thing and my conclusions are completely
wrong.