score:6
Tipping culture in North America is a nightmare. It's important to remember that all tipping is optional, and there is no "correct" amount to tip. At best there is a "normal" tip (i.e. what most people tip) which is made more complicated at one end by the restaurant industry constantly publishing articles trying to convince you that you "should" tip very highly, and at the other by the much-publicised fact that many wait staff don't get paid much and rely on tips for their income. (It's true of course, but restaurants will try to convince you that it's your job to make up the shortfall in the wages they are paying their staff).
If I rented a booth for $100, and during that time a waiter did nothing except bring me a $3 drink, there is no way that I would tip $15. I might tip 1 or 2 dollars (on the grounds that rounding up to the nearest dollar is usually a good thing on a tip). If the server did other things like bringing me golf balls I might consider more.
In short the tip should be proportional to the actual service given. Tipping on the cost of the food brought is a pretty good rule of thumb, and adding a bit more if extra services were performed, like them coming to ask you if you wanted anything at frequent intervals.