900 mg?! I wouldn't be going anywhere near that dose unless I was sure I knew what I was doing. DXM is not a drug to fuck around with thinking it's a cheap buzz.

As for your taking 700 mg and barely feeling it, what I'm wondering is: did you ingest the DXM on a full stomach? My friends and I have had many a wasted trip from doing this. You need to dose on an empty stomach. 700mg is more than enough for a third plateau with your weight. I recommend a bottle of Robitussin Maximum Strength on an empty stomach.