The dose will vary depending on what you want out of the trip I guess, and what your definition of "good trip" is.
To give you a guide, check the FAQ on erowid for descriptions of the effects at each "Plateau" because they differ quite a bit. IMHO, I like to slowly increase the dose and enjoy each part of the experience: 30-50mL of DX syrup (which gives a DXM dose of 90-150mg) is (imo) a mellow first plateau dose which can be social, a little euphoric, and a good body buzz.
If you dose a bit higher and hit 2nd plateau (each person's doses kinda vary so I won't list doses for the 2nd 3rd 4th plateaux), you'll know it, because your eyes will cross and you'll be in double-vision land for a few hours. Second plateau, in my experience, is less euphoric, less social, and more introspective - some visuals, bit more trippy in the head, but mostly able to be aware of what's going on around you still, besides a heavy body load.
Third and Fourth, you can read about in the FAQ.
There's tons of good info there.
BigTrancer
PS: Just to reiterate an earlier point, please never combine DXM and MDMA.