I can't answer that question, but my solution to it is to get a whole bunch of San Pedro cactus, chop it up, dry it out, and then extract it. Make a kilogram batch of dried powder from a variety of SP and then you'll be able to standardize your dose. I have found it is difficult to gauge the effectiveness of even cactus extracts that were cleaned up to a decent level of purity. My habit has been to dose liberally.
Last week I went to a local cactus vendor who has acres and acres of cactus and the winter did a job on many of his San Pedros. They were all dead and dying. I told him that I would like to try to revive the dying SP and he gave me a ton of free SP. In one part of the field there was a giant garbage heap of dead cactus with a lot of dead and wilted SP. Needless to say, I walked out of there with the mother lode! I am set for a while.