I base the tip around how our server is with us. If he/she has been particularly kind to us, I'll leave a tip around 20 - 25% (if they're absolutely wonderful/members of the family). I had an absolutely terrible time a few weeks back, and we left about $3 for a bill of $60. Ironically, the restaurant closed down a few days back.
When it comes to groups, if you split the bill, you can tip him/her 15 - 20% of what you paid, if you'd like. Although, the bigger the group, the nicer you should be to the waitstaff, especially if they went out of your way for you.
But for the most part, I'd base the tip off how well the service was when it comes to how the meal was during your evening.