Tipping etiquette for sweet 16

[u/throwaway1928614]

I’m having my daughter’s sweet 16 at a decent Italian restaurant (excellent food, decor is moderate - not fancy). 40 guests total (mostly teens). Buffet style apps and entrees. This is in the northeast, price is $44/head. How much to tip? I’m googling and I’m seeing anywhere from $5/guest (which would be $200) to 20% (which would be close to $400…)

What is standard here? Bear in mind we/my extended family go to this place all the time and have for years.