Like Mew said, potency varies highly based on many variables. Swim has a garden of about 30 different Trichocereus specimens, mostly from varying sources, and have found some to be super potent, while others have been not disappointing, but not what Swim was expecting at all. Different alkaloid makeups make for very different trips.
On a side note, be careful ordering just powder off of the internet. In powder form it is actually pursued by law enforcement at times. If you want to be safe, only order cuttings of cacti. They are perfectly legal to own and purchase for ornamental purposes, but ordering the powder form can be a bit sketchy. I have a friend who ordered some powder, and his package was held at the post office with a note left on his door for him to come get it. He was picked up by the local narcotics department when he went to retrieve his package :/. Be smart, stay safe.
Swim also finds that extracting down to pure mescaline left him wanting, and he missed the other alkaloids he had removed. Nowadays he just dehydrates the green skin and powders it. He either does an extraction down to all the alkaloids and ingests that, or simply washes down the powder with some water.
In reference to fasting, Swim usually also employs P. Harmala with his Cacti experiences, so he adheres to a strict no Tyramine MAOI diet and usually only eats fruits for the day before.
Also, Swim disagrees with the ice cream. He too thought it would be easy to handle, but was very disappointed and ended up wasting 20 mg of powder as he could only get down about half of his gloopy drink. He thinks you should be careful on what you put into the milkshake if you are going to try this method. He used vanilla ice cream, mango, milk, and a dollop of yogurt. He ended up with a cacti flavored yogurt, 2 cups worth of brutal tasting goop when he should've just spooned back the powder. Just his humble opinion that the milkshake idea was more difficult than just powder on its own.
Good luck in your travels, peace.