If I may make a recommendation...
Passengers served as a function of Stewards: P=(2^S)-1
1 Steward: 1 Passenger
2 Stewards: 3 Passengers; 1 Steward / 1.5 Passengers
3 Stewards: 7 Passengers; 1 Steward / 2.3-ish Passengers
4 Stewards: 15 Passengers; 1 Steward / 3.75 Passengers
5 Stewards: 31 Passengers; 1 Steward / 6.2 Passengers
6 Stewards: 63 Passengers; 1 Steward / 10.3 Passengers
7 Stewards: 127 Passengers; 1 Steward / 18-ish Passengers
2 more things: On larger ships, at some point, you're going to need dedicated chefs and bartenders, that this metric doesn't count. Additionally, at some point, it makes sense for the passenger to pay extra for an exclusive, dedicated Steward, which aren't included here either.