You should know both your fuel burn for the trip and also the distance.
Convert your fuel burn into gallons. Say 1600 gallons.
Divide this by the number of passengers. Say 230.
This is c7 gallons per person, say.
Convert the distance from nautical miles to statute miles. Say 1000 miles.
So each person would use 7 gallons to go 1000 miles. Thats 142 miles per gallon.
You just need the basic conversion rates and a calculator. Check it out on the next long flight, it is always roughly the same.