I am not aware of a baseline study which sets the standard for successfully
calibrating a who gets what out of an exhibit, much less determining what
a desirable percentage of comprehending souls would be.  It seems to me that
given the huge range of exhibit topics (from Vermeer to Star Trek Science) it
would be difficult to have a "one standard fits all" sort of evaluation.
The assumption that a museum presents an exhibit that ties in with its mission
statement is less of factor these days as "blockbuster" potential becomes
the guiding light.  Two people whom you might want to contact are Marilyn Hood
of Hood Associates in Columbus, Ohio, and Deborah Perry, of Selinda Research
Associates in Chicago.  Both are involved in evaluation of museum exhibits
and programs and Ms. Hood in particular has done pioneering work in the
field.
Good luck!
Anne Arenstein
Director, Public Programs
Museum of Natural History and Science
Cincinnati, OH