I worked in the restaurant industry for years. The tip is supposed to be based on pre-tax. I over tip as a general rule and use the total with tax as a guide line, depending on the restaurant, quality of service and the time I visited.
Good service should be between 15 & 20% .The easiest way is to double the tax amount, 9 times out of 10 you'll be in the range.
Fine dining, they get 20% on pre-tax. Casual restaurants depend on the time of day. If I'm visiting on the off rush hours they get 20% pre-tax if the service is good. Mom & pop diners are my exception & get anywhere from 15% to over 20% on the total with tax depending on my dining time. If I'm at a local joint during rush, it's 15% on pre-tax unless the service is exceptional.
My reasoning for being generous in the off rush hours is this; I worked multiple restaurants that were chain or individually owned. Individually owned restaurants normally do not have a dishwasher or bussers on hand during the off times. The work in the back of the house still needs to get done. Servers and kitchen associates end up taking up the slack. This also happens at chain restaurants even during rush times when not properly staffed by management and/or the restaurant doesn't have enough plates and silverware to handle the covers.
I rarely contribute to tip jars. Sorry Starbucks, but I'm not going to hand over money for a latte that takes 20 minutes when there are no customers in the store or the drive-thru. I can make a latte at home in 5 minutes. Not going to do it for any counter service restaurants either.
My exception to tip jars is when the restaurant is locally owned and I'm helped by someone who isn't the owner.
When shopping a restaurant, I always follow the guidelines. If the guidelines are ambiguous, I contact the scheduler and/or a supervisor to make sure I'm doing it right.
Live your life in such a way that when your feet hit the floor in the morning; the devil shudders...And yells OH #%*+! SHE'S AWAKE!