Actually, since finasteride inhibits 5-AR directly, the amount of substrate (Testosterone) may not matter much. And I don't believe finasteride and Test compete for the same binding site of 5-AR. So the predominant factor here is the concentration of finasteride.
It's like it doesn't matter how many cars are trying to get through a toll booth. If you put a gate at the booth, the flow of cars passing through the booth is controlled.