This drink turned out quite tasty, not unlike a non-alcoholic frozen strawberry margarita. I didn't measure anything, so I'm guessing at quantities after the fact. Next time I'll measure the ingredients and come back to revise the post with more specific amounts.
Pour into blender approximately:
1/2 cup tapioca drink base
1/4 cup of any other fluid (I used rice milk--you could just use more tapioca base if necessary, or leave this out altogether)
About 4-6 ounces frozen strawberries (maybe 8 or 10 berries?)
A few tablespoons or 1/4 cup grapefruit juice, or a quarter of a fresh peeled grapefruit.
The equivalent of 1 to 2 tablespoons sweetener (I used 1 packet of stevia powder plus 6 drops of liquid stevia). The amount will depend on your tastes and the tartness of the fruit used. I recommend starting with no sweetener at all and adding sweetener to taste only if needed.
Process the drink until smooth, serve and enjoy!