dude take the whole dose. cactus isnt nearly as strong mentally as shrooms and acid, but its top notch for visuals. Ive taken massive doses and been hallucinating so hard the patterns took over everything and it felt like i was in the middle of a brightly colored tornado, and another time my friend was smoking a when he blew out a hit the smoke turned into a gargoyle thing and flew away. all while being nearly completely lucid.
however it does taste like a mix of stomach acid and soap.
boil the shit down and freeze it into cubes that you can swallow like pills its a lot better then just drink something warm after.
seriously though a good san pedro trip is the best drug ive ever taken.