I hate to beat this dead horse again but is the service fee the tip or not?

We are on VG fight now. Last night at dinner, the menu read "a 15% service fee is added to the check" and then additional wording about gratuities appreciated. I wish I would have taken a picture so I could get the wording right.

We are not cheap tippers by any stretch, i was a server myself. That being said, I also am mindful of my money.

My thinking is that it is the tip and we add another 5% or so to bring the gratuity to 20+%.

Another opinion in my group is that the 15% is NOT the tip but part of the bill and we should be tipping on TOP of the food/bev and the service fee.

On a $400 dinner tab, the difference is significant. We eat out every evening so it's something we want to get right.

Thoughts on how to tip?