The thing that bugs me about tips is the fact they are a percentage. Why am I supposed to tip more if I ordered something more expensive but the service is equally good?
I agree if you had the $200 sea food tower by yourself it deviates from that otherwise perfect proxy. But that's already a weird situation so just pay the tip and move on.
Likely certain single malts vs jim beam.
Also, why do you need a proxy for the number of people when you can just count them? In Europe a service charge is relatively common and it's a per-person fixed charge.