What do you do about tipping on a bottle of wine?

Recently, at dinner, I ordered a fairly expensive bottle (1998 Far Niente Cab)and put it on a seperate tab as I would never submit this to my employer for reimbursement. So the tip on this tab was really nothing more than a corkage fee, however, I'm sure the server still gets taxed on 8% of the sale. I ended up tipping $25, less than 15%, but feeling like it was way too much.I expect the server was unhappy as well. What do you do in this situation?