Birdwatcher is right.
Now, to answer the question "how would you have tipped?", the answer is: Since 1987, French restaurants and bars have the obligation to include service in their prices.

The mention of "service non compris" is misleading. Sorry to disagree with you Iall but the bill is not legitimate on this aspect.

If I was very pleased with the service, I would have left an extra $3. The reason being, I tip 20% in the US when the service is excellent so I do the same there.

The term "pourboire" in France aka "for drink" in English, means "have a drink on me". That's exactly the logic behind leaving a few coins on the table. Nothing to do with service which is already included in the amount charged.

By French law, the waiting staff must be paid a living wage by the employer and not depend on the generosity of the patrons to make a living, that's why the price charged already includes the compensation of the waiting staff.