This is probably too late but never ever mix DXM with mdma no matter the dose, it can cause serious health implications, some dealers have cut there mdma with DXM causing people to overheat, dehydrate themselves and pass out.
Also if you plan on taking mdma to go to a club to dance or party or something then DXM is one of the last things you want to mix with mdma, I've read about people having a good time on about 150 mg of pure mdma and about 50 mg of DXM cause it can make you feel lighter but other people have reported passing out and becoming dehydrated, so it's not really something I would recommend since the cons outweigh the pros.
Always test your substances because research chemicals can have very adverse reactions to different drugs
And if you ever wanna find out good or bad combinations of substances then you can look at http://wiki.tripsit.me/images/3/3a/Combo_2.png