Since the total round trip cost is $85.00 and 12.75 is 15 % of that, 12.75 per leg would be a 30% tip. That being said I usually tip 10 a leg as well. I generally tip 20% and that comes out to about 8 dollars a leg. I round up cause it's just easier, but if I don't get great service, I would give less than 10.