Start low, and work your way up. 50mg is a powerful dose, and the difference between 40 and 50mg is pretty big, from what I've read. I usually go for 15-25mg, depending how far I'm willing to go, but everyone should find they're own sweet spot. Always weigh your doses. What you think might be 40mg might as well be 60 or more.
Doses over 50mg make you forget the experience, by the way. Take a look around the wiki and the forums, there's a LOT of quality information lying around.
The truth...lies within.