I find dose over 1000 mg to be unpleasant (too fucked up/disoriented to enjoy the trip)
But my very favourite dosage is exactly 900mg (on empty stomach, meaning 4hours after your last meal)
No alcohol !!! (not even a single beer with such a high dosage)
If you're using pure DXM powder (absorbed much more quickly by the stomach than DXM OTC gellcaps) then do it this way: swallow a 200mg pill, wait 20-25minutes, swallow another 200mg pill, wait 20-25 minutes and swallow a 250mg pill, wait 20-25 more minutes and swallow the very last 250mg pill .
....if you don't space the pills this way and take the whole dose (of pure powder) at once, then you will very probably get an unpleasant come up, too intense with too much side effects which could ruin the all trip (especially the euphoric & dreamy parts of it).
Enjoy the ride
900mg is a wonderful dosage, much less confusing than 1000mg but also much much more mystical than 600-700mg.
Better do it at home, by night (with moderate room lighting), with two friends (beeing three people is the best for dxm, allows you to sit down into a small circle and feel energies running between people).
And the essential thing is to play some trippy chill-out mystical music :
the 2 cd compilation called "spiritual worlds" is the best I have found for dxm trips...playing it for the first 3 hours always push me in a completely mystical state, truly beautiful (usually people don't even wanna change the cd for the all trip long as it's just the perfect sound for the dxm space).
Avoid watching tv for the first 4hours as it would ruin the spiritual part of the voyage.