It depends on your individual chemistry/metabolism. I don't metabolize it very well or something, so for me, when I was using opiates, I could take 400-500mg and be fine, in fact not even get a very strong high. No shaking or anything. But seizures are the biggest risk with tramadol and some people are really sensitive. My oldest friend was addicted to heroin on and off, and sometimes he'd go through tramadol phases when he got off heroin. Despite having a really high tolerance, he would only take 150mg of tramadol MAX... he said he took 200mg once and almost had a seizure, he was shaking uncontrollably and felt like he was losing consciousness and ended up taking a benzo to relax. He said 100mg was usually enough for him and that it was one of his favorite highs. Totally different from my own experience.
If you've never taken it before, start low, like 50-100mg. You can always take more next time, but if you're sensitive to it, you could die from taking 350mg or at least be in serious trouble. Tramadol is quite variable in how it affects people.