Birthday party event

So I’m throwing a big surprise party for my wife with about 50 attendees. This is a private room at a restaurant, open bar, food, etc. It will be about $3,500 and it includes an 18% gratuity. From experience with a similar situation in the past, when settling the bill after the end of the event, I’ve seen that there’s sometimes a line for an “additional gratuity”. Is it common/customary to tip above the already baked in 18%? Thanks for any advice!