I don't believe that data is available due to the lack of any real research into the effects of the drug, you would be looking for the LD50 value but ASAIK no one has done the testing.
When Meph was legal in the UK, or even now since its still widely available as a street drug a gram in a sitting would not be considered a massive amount, you should be fine, I assuming you mean .8g.
I would not take the Trazodone it is not quite an SSRI and I'm far from a doctor but Mephedrone messes with serotonin and I would be cautious about mixing in soem other chemical that acted on you brain chemistry in that way.
To my shame I have taken multiple grammes in one night and stayed up several days in a row taking more, this is a stupid way to use any substance but I'm just reasusuring you that you haven't over done it and should be fine, do you have access to and Benzos as they would calm you, the residual stimulation should start to ease pretty soon, try not to let anxiety get the better of you.
Feel free to PM me or post back I'm about now for several hours.
Best Wishes