Sorry there is no easy answer to this question at present. Unless taking a prescribed medication like Epidiolex or Sativex with precise doses to treat illnesses like Epilepsy or Multiple Sclerosis.
The industry is regulated by the MHRA and FSA and there currently isn’t any official guidelines on dosage. For supplementary use the amount required is down to several factors, body weight, individual chemistry and the strength of the product.
The best practice would be to start with a lower dose and gently increase it until you are able to reach an optimum amount. This may sound like unclear advice however it has been noticed that when treating anxiety, as the dose of CBD increases the beneficial effects become more noticeable until a point when the effectiveness starts to diminish.
Most clinical trials will administer between 25mg and 1500mg which is a massive scale, but it is worth noting that very few people experience negative side effects and if they do they are normally minor. Fatigue, diarrhea, changes in appetite and changes in weight are the most prevalent.