^^^
Higher dose shroom trips can be fun and immersive, but also more difficult to deal with if they turn bad. Take an amount you feel comfortable with based on your past experience with mushrooms, and be conscious of how the movie is affecting you. A Scanner Darkly is brilliant, but it's also pretty dark and I could see it feeding negative/paranoid thought loops if you start to fall into a bad trip. Obviously, if you notice this happening, you should stop watching the movie.
Happy New Years, enjoy and let us know how it goes