150-200mg is a good starting dose for dxm. You don't want to shoot too high; some people are extra sensitive to it or just don't like it, and if you're one of those people it's better to find out on a low dose than a high one. Once you choose a starting dose, a good rule of thumb is never to take more than twice the highest dose you've ever had before. 300-500mg is the general range for recreational doses. Above that and you'll start approaching 3rd plateau, which is more intense and less recreational. Generally speaking, DXM is only a "trip" at 3rd/4th plateau, and is just a bit trippy at 2nd (even 1st for some).