I can only tell you that Dr. John is of the opinion that DHT, as measured in the blood, is an unreliable measure of true DHT status. If you really want to know your DHT (at least in Dr. John’s opinion) you need to do a Rhein 24-hour urine panel.
However, I imagine that it really can’t be all that unreliable. After all, I believe all of the the studies we see showing finasteride knocking out DHT to 1/3 were using blood measurements.