If your intent is to use TS credits to order QS meals, you must have finished using all your QS credits.
Using TS credits for QS meals is never going to get you bang for the buck. You won't get anything extra, just the same thing you would have gotten for a QS credit.
When you get to the cashier, specify which meals you are using dining credits for and which you are paying for with another method.
Unless by DDP you mean Deluxe Dining Plan (usually designated here as DXDP or DXDDP) which does not have TS or QS credits, just meal credits that can be used for either; however, the QS allotment is the same as it would have been on a QS credit under the regular
Disney Dining Plan (entrée/combo and beverage, no appetizer or dessert included). Using those credits for the most expensive entrees possible is advisable, although you still wont get the most bang for your buck using deluxe credits for QS meals.