There is no 'set safe amount' for Datura for a number of reasons.
1) Every plant is different and depending on it's environment, it may produce a large number of tropanes or very little. There's no standardization, so it's impossible to say x-grams will be safe.
2) There's no data. Very few people experiment with dature and of those who do, even fewer are willing to go back and do analysis and bioassays at different doses to determine a dose-response curve.
3) It is so dangerous that very few people want responsibility for telling someone how to take it. Even the Erowid project, which will tell you how to safely cook meth and inject heroin, refuses to speculate on how much datura or brugmansia someone should take. Take a moment to think about that fact.