It might be a nice dose but why does it have to be that high the first time?
About 10% of the population doesn't synthesize an enzyme well (this is called a deficiency) which normally breaks down the DXM. If you happen to be among those people, the effects of DXM are different and much much stronger. So for the first time take a reasonable dose and stick to about 350 mg.
By moderating the thread I mean that we have a central "First Time DXM" thread, so this is not appropriate here. I understand your situation and for the time being won't do anything but this will eventually get added to it.