This is a problem with medical cannabis at the moment. Inconsistent. If you get herb from a dispensary, it should be labeled with THC/CBD content. But even that can change based on how you treat it. You'd need a degree in chemistry or pharmacy to know exactly how to manage all the variables.
So that leaves you as your own lab rat. Figure out what works for you through trial and error. Every person is different. It's gonna take practice.
Maybe in twenty years this will be solved, but in the meantime it gives us growers the power and responsibility to create exactly what we need.