I think the assumption that is being made is that 5-AR has more of an affinity for Testosterone than anything else in the body. If this assumption is true, then = good proxy. If it is not, then not such a good proxy, and taking more finasteride than is necessary is causing interruptions in other reactions within the body that could be prevented by taking a lower dose. My phd is not in biochemistry so eh I dunno.
As for the cost, I get 140 mg of finasteride for 39 dollars, an 8-10 month supply. I'm not trying to be rude, but if a person can't afford 39 dollars for almost a year of finasteride, they probably should be spending their money on other things like food? I was under the impression it was super expensive.
I don't understand that line of thinking. DHT is a byproduct of 5-AR activity, the percentage drop in DHT should approximate well the percentage of inhibited 5-AR II and III enzymes. The relative affinity for testosterone is get reduced should not be relevant, as you're comparing baseline vs post dose administration.
If 100 5-AR II enzymes produce 50ng/dl of DHT, then getting a serum level of 10ng/dl should give me around ~20 5-AR II enzymes, or a 80% inhibition, all else constant.