JFL at the cope, purely obvious Dutasteride doesn't have a 75% reduction In Androgen levels, DHT is only 4 times androgenic as T and T is 20x more abundant. Go read a book.Saying Dutasteride lowers your androgen levels by 20% is misleading. 5 AR inhibitors drasticly reduce androgenic stimulation. Your hormone profile becomes less androgenic and more anabolic. A purely anabolic hormone will not cause hair loss. There are actually two types of androgen receptors. DHT is the most androgenic androgen. Testosterone has a 1:1 anabolic androgenic ratio while DHT is about 5:1 in favor of androgenic if I remember correctly.
So say some one has, before dutasteride, 1000 ng/dl for T and 100 ng/dl for DHT. If we take DHT for having 5 times the androgenic stimulation as T, that gives and Androgenic value of 1500 and an Anabolic value 1020. After taking dutasteride for several months they get a 90% decrease ind DHT and a 20% increase in T. Now they 1200 ng/dl T and 10 ng/dl DHT with an Androgenic value of 1250 and an Anabolic value of 1202.
So dutasteride reduces androgenic stimulation by about 17% but increases anabolic stimulation by roughly the same amount. This is similar to your excercise in you did in your OP but you forgot to include the T increase with dutasteride. Also, this is not accurate because it's taking a systematic look at these values which is irrelevant when we are actually concerned with peripheral tissue levels.
From https://en.wikipedia.org/wiki/Dihydrotestosterone
So for every deciliter of blood that goes through the prostate, 90% of the T is converting to DHT due extremely high lvls of 5AR. So pre dutasteride in the prostate would look like 100 ng T and 900 ng DHT per deciliter of blood flowing through the prostate. After dutasteride, DHT would drop down to 9 ng (dutasteride inhibits 99% of type 2 5ar which is what is in prostate and hair follicle) and T should shoot up to around to 1110 (810 ng that were prvsly converting to DHT + 100 ng that wasn't + the 20% increase of T caused by dutasteride).
So pre dutasteride, androgenic stimulation in prostate gland per deciliter comes out to 4600 (900*5 +100) and after dutasteride androgenic stimulation is down to 1155 (9*5 + 1110).
So dutasteride reduces androgenic stimulation by about 75% in the prostate gland. These results should translate over to any tissue that has high levels of 5AR type 2, like the hair follicle. Much better result than your erroneous OP.