Finasteride (Proscar, Propecia,Prosterit) is an antiandrogen which acts by inhibiting type II 5-alpha reductase, the enzyme that converts testosterone to dihydrotestosterone (DHT). It is used as a treatment in benign prostatic hyperplasia (BPH) in low doses, and prostate cancer in higher doses. It is also indicated for use in combination with doxazosin therapy to reduce the risk for symptomatic progression of BPH. Additionally, it is registered in many countries for androgenetic alopecia (male-pattern baldness).
For use in cycles with testosterone one simply doesn’t want less DHT than normal. As the amount of testosterone in the system increases, the amount of finasteride needed to keep levels down to normal increases. I consider 5 mg/day reasonable at the gram per week level, and proportionally less at lower dosages of testosterone.
It needs to be noted that there are actually 2 different 5-AR enzymes, and Finasteride specifically blocks the type-II variety. The type-II 5-AR enzyme is the one responsible primarily for hairloss and prostate enlargement, while type-I is often the culprit behind acne and hirsutism. In either case, type-II is responsible for around 2/3rds of the circulating DHT in your body, so it's no surprise that Finasteride typically reduces your total DHT levels by around 65%.
If you are worried about your hairline, or have incidences of prostate issues in your family, then 1mg/day of Finasteride may be the answer you've been looking for. It's not recommended to exceed pharmaceutical dosage, or 5 mg/day. More might not be safe and might at some point cause DHT levels to become abnormally low.