My aunt endlessly researched all the options at Starbucks and then finally came up with this gold standard. (She writes out on a post-it for every visitor to her home and sticks it in their wallet so they can recite it in front of the barista.)

Cafe latte, tall, non-fat milk, one splenda. (This is fairly zero calorie.)

A stronger option is: Cafe latte, tall, an extra shot of espresso, non-fat milk, two splendas.

Ask for it to be made with Reserve beans.