Tipping etiquette

I'm always scared I'm offending someone by not tipping adequately, or not realising theirs is a tip-worthy service. I stumbled upon this tipping guide, and now I know exactly how much I should be slipping the takeout delivery guy, as well as what to do at funerals, hospitals and the country club. This bit takes the cake:

"If you hold a table for two serving periods, make sure that you tip
double. In other words, if you spend enough time at a table that a waiter could
have typically gotten two parties seated and served, then compensate him for his
time by tipping him twice. I like to ease his mind by telling him this about
half-way through."

If you're sitting there for that long you're probably onto your fifth bottle of wine, and surely the 15-20% of the bill gratuity covers the tip adequately! Seriously!