In article ,
"Neil Gould" wrote:

WHAT! If there are 4 the pro rata share of $100=$25. If there are 100
the pro rata share is $1.

pro ra·ta (pro ra't?, rä'-, rat'?)
adv.
In proportion, according to a factor that can be calculated exactly.

Your generalized application of the term "pro rata" does not account for
the 50% requirement. For example, one could easily "calculate exactly"
20% of the cost of a flight, but if that is all a private pilot pays, then
the FAA is likely to consider the other 80% paid compensation.

where is this "50%" requirement?

--
Bob Noel
(goodness, please trim replies!!!)