There needs to be a lot more changes in the brain than just high dopamine for someone to have schizophrenia. Many people would have elevated dopamine and not be schizophrenics. For example, impulsive and aggressive people have high dopamine but are not schizophrenics. Happy people have high dopamine too!
https://my.clevelandclinic.org/health/articles/22581-dopamine
Moreover, high dopamine in schizophrenia is characterized by it being high in particular areas and particular cells, not the whole brain. Schizophrenia is also categorized by misconnected brain regions, cellular type imbalance (e/i ratio), misregulated synapses, and more! Dopamine alone is not enough to categorize schizophrenia or ADHD!
https://www.healthline.com/health/schizophrenia/schizophrenia-and-dopamine#:~:text=Increases%20in%20dopamine%20activity%20in,factors%20involved%20in%20schizophrenia%20symptoms.
Because mental illnesses are poorly understood and share a lot of commonalities between them, psychiatrists often diagnose by symptoms and expertise as opposed to biological changes. We do not know enough biology (yet) to look at a brain and say schizophrenia or adhd; we look at a person's behavior and symptoms, etc.
DSM 5 is what psychiatrists use to diagnose: if you look at the section for schizophrenia or adhd, there is not much biology, just behavior.
https://my.clevelandclinic.org/health/articles/24291-diagnostic-and-statistical-manual-dsm-5
It is not possible to calculate the probability of ADHD and schizophrenia from dopamine alone because dopamine misregulation in schizophrenia is region-specific and needs a lot more brain changes with it. Also, there is no accepted way of biologically differentiating schizophrenia or ADHD; it is only (right now) by the person's behavior and appearance etc.