of a tab? umm, it depends how strong it is. the first time i tried lucy I had 3 quaters and saw mermaids, hindu gods and dinosaurs. it was full on. maybe a quater or half. and green.
Shrooms? Well if it is common shrooms (Cubensis/Liberty Caps), I would say do a full eighth. That's what I did my first time and that's what i believe one should do in order to really feel like they are taking a shroom trip. Like, it's not insane, but it's not weak.
Eat the whole thing, if you are splitting the eighth between a friend and yourself, then fine, but if you are trying to get the most out of it, eat the whole thing.. I've tripped mushrooms a good bit, and everytime I'm with people who eat half their eighth, they seem more likely to have a bad trip, or one with just a mind fuck and no visuals...which is not fun. I still vouch for an eighth being the 'ideal' amount of mushrooms to be consumed. Have fun
Snocbar hasn't run in to bomb ass shrooms yet. When he does, and 1/8 will make him cry. Stick with half an 1/8. You can eat more two hours later if you need to. Have fun.
Oh you know me? I "run into" bomb boomers all the time. A full eighth did make me cry, twice. They do vary in potency a LOT but I still vote full eighth being the most ideal.