Good Question.
Nobody has the correct answer but I can give you some sound advice.
I am extremley prone to MPB and started losing my hair at 19. I come from a high mpb background. In fact my father was completley bald at the age of 25. After being on 1.25mg of finasteride(proscar), I started seing a reduction in fallout after 4 months. I didn't grow anything back, but it definatley stopped or severley slowed down the balding process.
Since then, i have started AAS and barely lost anything. I've even doen 600mg of deca/wk with proscar and no fallout. Remeber I started losin it when I was about 19. I am now 26 and an amateur competetive bodybuilder. I have gone as high as 1 gram of test/week with 40mg dbol and no intense shedding.
So...heres my advice to you......If your taking 500-1000mg per week of test, up your proscar dosage during the cycle to 2.5-5 mg per day and you should not see any increased fallout. Hope this helps.
Good Luck