My personal only precaution when it comes to psychedelics is, don't take too many precautions!*
I feel like some of the advice on the Internet regarding how to prepare for trips is counterintuitive to the experience psychedelics are supposed to create. The first time I dropped acid, I had done so much research on how to have a good trip that I set myself up for a "bad trip," but both during that trip and in hindsight, it wasn't really bad. Just a different experience.
Unless you have a specific goal you would like to achieve, like a creative project, or something, just go with the flow. It might be nice to have some starting point planned on what you would like to do at first, because if you have nothing planned to do, you might get the urge to do something but you won't know what. If you're just tripping in your home, maybe have some coloring books on hand, finger painting, different foods to taste, music, etc. It's good to have options easily and readily available, but don't get too caught up in a set plan. I think it's best to just let the trip guide itself. Keep in mind that psychedelics heighten all of the senses, so overstimulating activities (i.e. riding rollercoasters...personal experience, lol) almost cancel out the perception of the drug. Simple, methodic, rhythmic activities are what really seem the coolest and let you get the most out of the drug.
*When I say don't take too many precautions, I'm referring to the laundry list of set/setting precautions that you could find on the Internet. As for safety precautions, those are good to take. I.e. don't put yourself in a potentially dangerous situation (like going for a walk at night alone if you don't live in a safe city); your perception will be hyper-focused and not in line with reality, so it's not easy to react quickly and carefully when you need to. And because your perception is hyper-focused on sensations, you can lose yourself in the feel and forget to take care of yourself. (Ex. if you go out for a walk in the dead of winter, you can shift your focus away from the cold and stay out there too long, which can cause lockjaw later on and an achy body...plus the threat of frostbite if it's cld enough and you're underexposed!) Common sense doesn't seem so common on psychedelics, so just remind yourself and ask yourself here and there if what you're doing is safe. Other safety precautions would be minding the substances you're using. If you were to combine any drugs, even something simple like a Xanax for coming down, research first to make sure it's safe. (A lot of people recommend against drug combos, especially for a first time experience, but IMO, there are safe ways to do certain combos.)
If you want to avoid "negative" emotions like fear, just remind yourself that it's all in your mind, and you control you mind. You are impervious to fear if you want to be. But should you experience an emotion like that, think of it as a color in the spectrum of emotions, and an experience worth having.
Also, like already said above, 400mic in one tab sounds unheard of, but if that's the truth, that's way more than enough. Larger doses can increase the intensity, but IMO, they mostly just lengthen the trip. I'd say take a half to start, and redone later on if you so wish. I think starting around noon is a fine time to start too so you can experience a little of the day and the night, and probably be able to fall asleep at a reasonable time too.
good luck