Accurate dosing of mushrooms and other non-synthetic psychedelics is very difficult anyway, as the active compounds can greatly vary even within the same species and even within the same batch.

Also, with psychedelics the effects will vary from person to person, and even on the same person depending on that person's mood, body chemistry, how much and what they've eaten, what's going on with their life and their subconscious mind at the time, what their expectations are, their stage of development, where and with whom they take it, etc.