Years ago I waited tables. I hated it because my income was highly variable. I decided that I would prefer getting exactly $4.75/hr (whatever minimum wage was back then) at McDonald's instead of my pay being a random number. McD's offered more hours so I ended up making about the same amount of money without having to dance for nickels.
Variable income is fine if it's a lot of money -- actors, musicians, authors, consultants, etc. But when your variable income is not that much more than minimum wage, it makes no sense. What makes even less sense is making the server share tips with other employees.
Let's take this to its logical conclusion: advertise "FREE FOOD!!!" and then tell the customers that they need to tip an appropriate amount for the server, the other employees, the wholesale cost of food, the building mortgage/rent, the electricity, etc. A percentage method clearly won't work, so produce a receipt with zeroes on it for the order, and at the bottom a suggested tip range which is a calculation based on what you ordered. What used to be a $200 dinner with an expected tip of 15% is now totally free with an expected tip of $230. The servers have to tip out the busboy and ConEd.
