My point was that we can't know what a lethal dose of fentanyl might have been for Floyd unless we also know how much he was using & for how long. The body develops tolerance to opioids. What might be a lethal dose for you & me might not have even got Floyd high.
And yes, I am, or was, an expert. My first career was nursing & I worked in psych, rehab, in-patient heroin rehab & outpatient methadone clinics in my career.
But knowing that the body develops a tolerance for opioids with regular use is basic general knowledge.