It's 12.5% btw. I've gotten so used to 15% as a minimum that I automatically assume that now. That's my bad, I was wrong saying 15%. But 8% is straight up wrong.
1) Claim 12.5% of all sales or
2) You claim all credit card tips and claim cash tips within 5%. So if you make a 20% CC tip, the government assumes you can make at least 15% cash tips.
When I was a waiter, I was aware of 12.5% only.