As I outlined it, the banker must declare what the tip shall be (or perhaps declare tax + tip since most people will remember the menu-prices of their items) and the table gets a chance to argue it up or down, but one it is set, the expectation should be that any fair payer must not stint on this. Though I would have thought you could allow people to go up or down from a base tip.

Now, if your declaration that it doesn't work (in your situation) is true, then the banker is indeed taking a risk, and so having the rare profit go to the banker does not seem unfair, if they're taking the more common loss. But yes, the system is that the banker is the one person who doesn't calculate what they owe. They just pay what it say son the bill, plus the tip they declared, and don't count the money or calculate their own meal. That is the reward/punishment for being banker.