If you plan on doing that, your best bet it to figure out how much each meal would be likely to cost OOP, and of course pick the least expensive ones to pay OOP. Breakfasts and buffets are almost always the cheapest -- but really check your figures on any 2 TS restaurants, because
per credit, they might be the smartest ones to pay OOP. When I did the math on all of our ADR's, our least expensive meals will be breakfasts at Boma and CP. I have 9 TS planned and 7 TS credits for each of us, so those are probably my OOP ones.
One word of caution -- actually a FEW words -- last year my DH got horribly sick about 2/3 of the way through our trip

and we missed our last two TS meals because of it. We had paid OOP for the plan, and fortunately had gotten such a great value up to that point that we really didn't lose money, in fact we still came out somewhat ahead -- but that was back when appetizers and tips were included

Even if no one in your party gets sick, (and I wouldn't wish that on anyone on vacation!) there may be a time when you don't make it to an ADR, or aren't hungry, or are in the mood for CS instead. So just be mindful of banking your TS credits too early in your trip so you don't end up wasting them.
