I'd go with the Robitussin cough gels (or better yet, the cheaper generic CVS version). One bottle of those generally has twenty 15 mg gel capsules, meaning that one bottle is 300 mg, which was always enough to get me just barely into a 2nd plateau. I usually preferred to use between 30-40 gels (450-600 mg), as it takes closer to 600 mg for me to feel truly dissociated, but you shouldn't go for that much the first time. I prefer the gels because I have a better nice of getting them down without throwing up - chugging syrup always ends in me bent over a trash can. That, and after drinking that shit so much in high school, I can't stomach the taste of it anymore. The gels are easier on your system since you're not drinking all those sugars and whatever-the-fuck else they put in those syrups.