No, I just pulled 0.5% out of my ass, since I didn't know the actual goal of adding this thickening agent. I don't know what your target viscosity is. This is a really powerful thickening agent. If you go a little bit too high, you will have inedible toothpaste or spackle. Honestly, I think a smoothie with peanut butter & banana is fantastic, and I don't exactly understand your goals.
But to answer your questions, it's more typically added as 0.8% of the total weight, not 0.5%. You should include the weight of the water in the calculation. Whoever you spoke to was correct, and they may have other good suggestions for you.
Get a scale accurate to 0.1 grams (I use the Weighmax), and also get a calibrating weight. I think that model requires a 500 gram weight. If you've never taken chemistry lab, you might want to experiment for a while without using expensive ingredients, e.g., add beans to dry rice so that you have 0.8% weight.
When I cook with it, I either measure everything extremely precisely, or I'm adding a pinch at a time, then using a hand mixer.