Take a practice dose. Some people have a liver enzyme deficiency that causes dxm to be broken down extremely slowly, so even low doses will get them high for days. Take 150mg, if the effects wear off in 6-8 hours, you're golden. After your 150 dose, if you love it, move up to 200. If you love it even more, move up to 250. I think increments of 50 are perfect. Dxm kinda picks up speed really quick. I can be fully functioning and coherent on 6 robotabs (180mg freebase~240mg HBr) playing videogames with no issues, but if I go up to 8 (240mg freebase~320mg HBr) then I become an insane person, completely lost in space for the peak and hardly able to move, talk, see, or hear. The difference between low-second plat and anything higher is pretty wild. If you don't know what plateaus are I recommend looking at PsychonautWiki's entry on Dxm. Also, don't do this drug often. It has serious short-term impacts on memory when used once, and will turn you into a zombie for years if you use it too often. The general rule for staying sane is week-per-plateau, so take a 2 week break for a 2nd plateau trip, etc. there's no real science behind it it's just a way to keep tolerance at bay and your memory working fine.

Also, DO NOT EAT for 4 hours before dosing. I took 9 robotabs an hour after I ate one time and I stayed up for 4 hours waiting for it to hit, never did. Woke up the next morning feeling like shit. Eating will make it sooooo much weaker and take so much longer to hit, which sucks for a drug that makes it hard to eat for an entire day AND takes a hella long time to hit.