You can add 2 packets sugar substitute, if desired.
I like my smoothie thick enough to feel substantial in order to replace a meal. I eat it with a spoon, like ice cream almost, therefore I only use about 4 ounces of water. If you like it thinner, add more water.
In order to cut calories, I sometimes use sugar-free peanut butter syrup to supplement all the peanut butter I'd like to use but don't. Have used powdered peanut butter (PB2®) also, it just depends what I have on hand.