You dont need time to do this because you could just for example leave the MHRB soaking in water over a couple of days instead, you dont have to be physically present as it soaks.
Sure maybe the "majority" of alkaloids, but would you be satisfied if you were losing for example 30% of your alkaloids? Or 49%? And even if you for some reason wouldnt mind wasting all those alkaloids, there's still the fact that you wouldnt know how much you are wasting so you couldnt accurately dose.
It makes much more sense to do 3 or 4x boils, to approximate as much as possible to 100% extraction efficiency (but more boils not necessary, because of 'law of diminishing return'
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. I can safely say from my experience that the 3rd or even 4th boil, depending how powdered, how much liquid and how long boiled, will still have significant amount of alkaloids. You could test this yourself, extract the third soak/boil separately someday and see what comes out.
As for your idea of dividing dosage, I cant understand it. The fact is, the alkaloids will be in the liquid, and the proportion of liquid you drink = the proportion of original start material. The more you reduce, the less liquid you need for a dose. It doesnt matter if it seems to you for some reason like a small amount cant be that potent, because it really can (unless your cooking method is indeed amazingly inneficient that only small percentage of alkaloids are there). So please be careful and really do drink a small amount for the first time, I dont want to read on the news a story of the dude that was arrested for drinking equivalent of 100g mimosa and running naked in town in a bad trip screaming about hyperspace dragons or something
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I really think you should do it properly next time in a less wasteful way and be attentive on the dosage.