"What is the ideal alcohol content for a reishi tincture?"
Well as we say in both permaculture and herbal medicine, it depends. Water and alcohol extract different constituents of herbs, so higher/lower proof depends on what you are wanting to extract and that usually comes from what the point of the final medicine is. For every herbalist who says reishi is best at x concentration, another will say they've had great results with y. There is also an issue about heat (heating can release/activate some parts of plants/fungi that wouldn't be otherwise). In general heat/long decoction will extract more immune properties, alcohol will get the adaptogenic properties.
If you are doing a double extraction, then the final alcohol% is going to be different i.e. you can make a 96% tincture, but not using a method that involves decoction as well.
I use reishi as a decoction, and haven't made tincture yet, but here are the recipes I would consult. They come from expert level herbal medicine practitioners (Michael Moore and Robyn Klein. Klein is a hardcore scientist, if that's the important thing).
www.swsbm.com/Distance_Learning/.../Lesson_7_text%20only.rtf
www.swsbm.com/school/files/quiz7part4.pdf
I'm really excited about my new mushroom hobby but, as a realist I would like to challenge everyone out there, mushrooms will not be able to save the world, much less me, with out a free flow of accurate information that the worker bees can use to proliferate this technology.
Again, as with both permaculture and herbal medicine, these things are both science and art, and both theory and practice. What I have found with both is that I can learn from the experts but I need to get my own experience to be able to make good judgement calls about what is best in any given situation. There are no absolutes, everything depends. This is a good thing btw. I agree with you about a free flow of information
To that end one thing you can do is make smaller amounts of 3 different tincures. One is a alcohol tincture. One is a decoct then tincture tincture. One is a tincture then decoct tinctures. Try them out, see what happens. Strongest isn't always best, and using herbal medicines is an exceptionally good way to learn more deeply.