I feel like I should know the answer to this question as a matter of
cultural literacy, but I don't:

When looking at the average price of entrees on a menu in the United
States, what percentage/multiplier should I use to estimate the cost
of dinner for X number of people?  Assume two cases: 1) Without
alcohol and 2) with alcohol.

Thanks,
The Grammaton Cleric

I think this would really vary by the city and type of restaurant, but
my guideline would be 1.45 multiplied by the number of people without
alcohol and 1.8 multiplied by the number of people with alcohol.
(Including 7% tax and 18% tip)