Obviously I'm no expert, but...when I got around to trying it, I did this:
1st time 25gm: mild but definite trip. Lots of energy, only positive thoughts, stayed up late looking at stars and planets with a sky scout. Very positive.
2nd time 50gm: stronger, and still very pleasant. Great time. Like mild dose of acid, but easier to stay positive and happy. (There's more to describe, but I'm not great at putting the feelings of a trip into writing.) Spent a lot of time hanging out and was able to have a lot of fun playing with 3 yo niece without freaking myself or her out. (I did boil over the tea for a few seconds this time, so I may have lost a little bit.)
The tea will taste disgusting. Worst than anything I've ever tasted.
You will also be nauseous for the coming up for about an hour. I believe this is a characteristic of mescaline and will happen even with extracted mesc.
The dried cactus was primarily the skin and just below it. If you dried more than the outer part, the potency/weight ratio will be reduced, so you will need more.
I would suggest taking it and having some time to lay around and not talk during the coming up. I just lay down and almost sleep for about an hour. Then slowly felt better...then great and the fun started. I personally get a little irritable when nauseous, and it'd be a bad start to get irritable towards friends just before a trip. Smoking weed would probably help, but may dampen the trip.
To prepare the tea, I ground, put in water to cover, added lemon and simmered for 2 hours. Poured off and strained through fine strainer. Save liquid. Then repeat and combine stained liquid. I then strained them through an old cloth after it cooled down a little and was easy to handle. Then boil down the liquid for a while to reduce volume.
Compared to mushrooms, I think they were much more positive and energetic. There was no confusion or scrambled thoughts that I sometimes experience on shooms. As I'm sure you know, it lasts ~12 hours and will keep you awake. I consider this a plus....with shrooms I always stop tripping too soon and feel a little bit disappointed.
I can't imagine you wont have fun unless you take too little and get the nausea without the trip. So, I'd agree with the last post: err by taking too much not too little. It seems to be a very positive, energetic, fun psychedelic.