Honestly $15 sounds a bit slim if you drank the entire night. (I guess it depends on what it takes to make you barely function ).

I try to tip about 20% if service is bad and 30%+ if the service is good based on the approximate retail value. So if the double shots are normally, say, $10 (been a while since I bought shots so my number might be a bit dated), I would drop $2 to $3 per go. A more current example for me would be in a casino on the strip. I was gambling in the Palazzo (part of the Venetian) last night and was tipping $5 per round of cocktails or...later in the night....coffee and water....for me and Dillwoman.

I've found, time and again, that servers in open-bar or any kind of free event figure they are going to earn less and if you tip ABOVE what is acceptable, they will pay you back with good service, perfect drinks, and a better time. YMMV