90% of people tip post-tax as far as I can tell. So, your server probably just thinks you're not quite as generous.

The way I look at corkage fees is thus: your server is doing all of the "service" of your wine; you're just not paying for the wine from the restaurant. So in my opinion, I would tip on the corkage fee plus some. Let's say for food etc., your bill is $100. The bottle you brought in might go for $70 on the restaurant's wine list. So, your bill would be $170. A 20% tip (in my post-tax world) would result in $34. Instead, you have a corkage fee of maybe $15. So your bill would be $115. I would think $25-30 would be appropriate. Again, this is assuming competent service. I'm the first one to advocate not tipping well if you don't receive a superior product. Keep in mind, servers "tip out" on the corkage fee too. So if you don't tip on that amount, the server is losing money on the fact that a) you didn't order wine from them thus keeping your total smaller, and b) they are tipping out on sales from which they didn't receive a gratuity.

I think the biggest misconception of the restaurant industry is that servers get paid by the restaurant. In all reality, we don't. In most states, we get $2.13 an hour from the restaurant. But, we tip out anywhere from 3% to 10% of our total sales, which is ALWAYS more than $2.13 an hour. We essentially pay to wait tables in a restaurant. Our "employment" at the restaurant is merely their willingness to have us "rent out space" in their building. So, if you don't tip, we actually lose money on whatever revenue wasn't tipped upon.

Chris