That is a lot of table service dining over the course of your trip. That takes up a lot of touring time, considering the average TS is 45 mins to an hour. And of course, if you go with the basic plan, 1/2 of the table services you choose (if you do 2 a day for your entire trip) will have to be out of pocket anyway.
Is the third party in your group a child or an adult?
But back to the orig. topic of tipping... Like mentioned, it is a reasonable amount if you are just doing 1 TS a day. Yes, the amount grows fast if you add more TS choices to your plans. The thing is that you are going to find that, for the vast majority of the time, the service will be very good to excellent and will warrant the suggested 18% tip. This is something that you really should include in your overall travel budget.
In essence, if your food budget does not have the provisions for the gratuities for the extra TS meals, you might want to consider some alternative, yet excellent, CS dining choices. Besides, after about the 5th or 6th day, you are probably be tired of all the food - especially if you go during the hotter months. (Been there, done that, and I am a person who loves to eat!)