first time i tried dxm was 600mgs so you should be fine. Just dont expect it to be a mild high because it can be bought over the counter, dxm is the only drug that has left me completely speechless, its honestly the weirdest feeling, completely different from lsd. You probably wont need a sitter cause you most likely wont be doing a whole lot on it anyway, but definitely prepare yourself for a serious mindfuck. I'd probably compare the strength of 300mg to about a tab of acid (the tabs i get anyway) Just chill in a room and put chuck on some beats, hard electronic sounds fucking amazing on lower doses of dxm, shit like dubstep, trance (the heavy, spintwist kind) etc, but whatever floats your pickle really. Body temperature can go a little nuts sometimes too so i'd do it at night time if i were you, also DONT eat a few hours before taking it or you WILL feel sick! I've done dxm about 30-40 times (almost all 600mg or over), and the only time i've ever puked or felt sick is when i ate beforehand.
Also just a sidenote, if you're drinking it I strongly advise downing it all in one big scull, its much easier than doing it little bits at a time and you'll feel the effects faster. Have fun :D