Does anyone know of  some guidelines or rules of thumb for predicting income
from a donation box placed in a small museum or exhibition space that does
not charge admission?    Can you estimate based on attendance and the size
or type of the museum.  One small museum I worked at averaged about .50 per
visitor another about a $1.00.    Another museum determined that one out of
every three visitors paid the "suggested donation"—which I guess means that
perhaps more than that paid something but they did not pay the full amount
suggested.