Chuck,
If you haven't settled on the $$'s yet, I'd tell the customer they need 3 ribs and 1-2 peices of chicken pp. You know how these things go, some will eat 2 ribs some will eat 5, some will eat only chicken. But as long as you talk it over with the client before hand, they won't blame you if they run out of food. And I'm sure they won't complain if there are leftovers.
In the words of that great BBQ philosopher Steve of Denver Cajun BBQ Crewe - "You never want to run out of food."
My BBQ calculator calls for 2.5 ribs pp. Only a calculator would come up with 2.5 ribs pp.
JMO...
Frank Eriksen
Roaring Fork BBQ