Your gratuities will be $168. This cover ALL of the people that you are supposed to tip. $12 pp/pd.
Does your $500 include what you will spend on shore? Do you mean that this is the cash you will need or will you put some purchases and expenses on your credit card?
How much someone spends is entirely up to them. The $100/day is a number that is put out so that people can enjoy the cruise without feeling they can't spend and so they will not freak out when they see a five or six hundred dollar bill at the end of the cruise.
You could easily cover your onboard cruise expenses for $500 and still have dinner in the specialty restaurant and have a drink or two a day. As others have said; The main dining room and entertainment is included in your cruise fare. If you deduct your gratuities it would leave you with $332 or about $50/day for drinks or surcharge restaurants.
If the $500 includes the cost of your shore excursions and other onshore expenses then you may not have enough. How you spend is up to you.