There is no way of knowing the yield of your cacti extraction. The concentration varies from one cacti to another, and one location to another, weather, and even how long ago the plant was chopped, as there is a technique I think they call it aging, to chop the cacti and let it sit around for 6 months and it gets stronger, allegedly.
What The-Hive posted was great, there are a bunch of teks online just Google acid based mescaline extract. They're all basically the same, extract, basify, pull into certain NP solvents, then extract into the aqueous and evaporate. The needles will form on the plates you use to evaporate.
Make sure you do multiple extractions, and if possible, freeze and thaw the cacti first to bust up the cells and make the 345 easiert o extract