OK, I've got quite a bit of experience with doing DXM and also introducing ten or so people to it. I wrote a Dutch-language manual on how to obtain and correctly use DXM that was used by a TV program about drugs. They would feature one drug every episode and have one of the show's presenters take the drug in question.
If you have not taken it before, take 100 mg or so to test for CYP2D6 deficiency, which roughly 1 in 7 people have. If you trip balls off of 100 mg, then you know your liver is missing (or lacking in) a certain enzyme that breaks down DXM (among other drugs). It'll still be metabolized, but through a longer, less direct route.
If you take a full dose of DXM (300mg or so for people with no tolerance who weigh about 75 kg/150 lbs), you'll trip extremely hard and for up to 24 hours, which can be very unpleasant.
So start out with 100mg if you're just getting into DXM. Otherwise, I recommend a 300mg dosage for a solid mid 2nd plateau trip. Adjust up/down for different body weights/metabolisms:
- Adjustment for weight in percentages; if you weigh 60kgs/120lbs, you need to take 80% of your normal dose (i.e. 20% less). If you're 90 kgs/180 lbs, take 20% extra DXM. You get the idea.
- If you know you're one of those skinny mofo's who can eat whatever they want and not gain weight, take 30-40% less or it will hit you like a freight train.