When you don't have tolerance, one 300mg can be enough to induce a drunken like state. Have you been prescribed these 300mg, and taken before? Then you could go for 600 or 900mg, I personally don't feel safe recommending more than 1g and be specially cautious if you ever had a seizure before, recently I took probably 1.6-1.9g (staggered so don't know for sure) and ended up seizing. With 1g+ I'd get strange paresthesia like mild electric shocks in my limbs, guess this was the warning shot.
For the same reason I would recommend against mixing with tramadol, because this one is known for lowering the seizure threshold. Little risk of serotonin syndrome, it's routineously prescribed together with SSRIs. But it's a nice enough drug when taken alone, just tolerance is from hell as you probably know when you took it before. Today 300mg, tomorrow need 750 for the same effect and so on. Just that as it seems, tolerance doesn't build to the seizurogenic effects :/