Recently, Ron Garret posted:

In article ,
"Gig 601XL Builder" wrDOTgiaconaATsuddenlink.net wrote:

Neil Gould wrote:
Yes, but that usage renders 61.113 (c) meaningless without some
additional parameters. One can calculate many things precisely, even
without the use of a calculator. ;-) That might not guarantee
compliance with the FARs.


Pro Rata means a specific thing. It is well defined in law and
common usage. In 61.113 (c)the regulation is quite clear. A pilot
may not pay less that his pro rata share of the allowable expenses.

There's still an ambiguity here. The obvious way to compute the
pro-rata share is to divide the total cost by the number of
passengers. But that is not the only reasonable way to do it. One
could, for example, assign a pro-rata share according to weight, so
that heavier passengers (or passengers with more bags) pay more than
lighter ones. One could likewise use height, on the theory that
taller passengers use more room inside the plane, or some combination
of weight and height. If all the passengers are owners of the company
for which business is being conducted they might choose to assign
their pro-rata shares according to their percentage ownership in the
company. None of these are unreasonable on their face, and none of
them are specifically excluded by the regs. But I wouldn't want to
be the one defending any of these theories against an FAA enforcement
action.

I also saw such ambiguities, based on the dictionary definitions of the
terms rather than their basis in law. However, the explanations by
AirplaneSense in other parts of this thread have cleared up the matter for
me. I now see that Gig 601XL Builder's comments are in agreement with the
interpretations of AirplaneSense, but were a bit terse to be enlightening.

Neil