I find the 2/3rd rule to be an excellent rule of thumb, but it is admittedlly not exact.
Of course, it must be remembered --- You likely won't break even if you simply use 2/3rd of your credits and throw away the otehr 1/3rd. You still need to at least get some qs value out of that 1/3rd. And likewise.. if your 2/3rd TS meals are breakfast and a moderate lunch... it won't put you towards breaking even.
But most moderate lunches will still start around $30ish... moderate dinners start around $35ish... (By moderate, I mean mid-level entrees... even possibly skipping a dessert or app).
So if you use 2 TS credits for lunch and dinner... You will likely be "spending" at least 65-70. So if you can get another $10-15 value out of a QS breakfast/snacks, mug... then you break even.
When we did the DxDP, we used 15 out of 18 credits for TS. There were days when we used 2 TS credits, (lunch and dinner), and days where we used 3 TS credits (lunch and signature dinner).
On days when we only did 2 TS credits... It was more or less break-even. (For example, 1 day, we did snack breakfast, Liberty Tree Tavern lunch and Chef Mickey dinner-- adults averaged about $75, slightly less than break-even.).
But days when we did a signature meal-- We were "spending" between $90-110 for the day. (The cheapest 2 credit meal was $53 at Hoop Dee Do, we had a $35 lunch at Yak and Yeti that day, and QS breakfast, so about $95 for that day).
To me, the easiest way to get value out of the DxDP is to mix in quite a few 2-credit meals.