Don't.
Americans, and to a lesser extent British, are uncomfortable NOT leaving a tip, but tipping for meals and taxis is not a custom in France and Spain. Even poorly paid service employees pride themselves on being professionals at their work, and they are paid the going wage to do it. If the VAT results in a few small coins in change, no one is insulted if you leave it. The ex-pats on the Board may be able to amplify a bit. At dinner with a French woman and a Quebecoise, we grilled a friendly waitress in a small French town on the idea of tipping, and she insisted that no one expects a tip.
Minimum wage for a waitress in the U.S. is $2.13 per hour (yes, $4,430 per year)!!!! No tip, no rent.