I'm prone to MPB and i've used HDrol several times with no hairloss.
HD A:A ratio is 2.6 (75/28) ->
http://anabolicminds.com/forum/steroids/89979-otc-hormone-chart.html
HD it's probably the safest PH to hairloss.
Epi is very harsh for hair, but it has a high A:A ratio of 12 (1100/91).
I don't run SD yet (i'll do it on January), but for the logs i've read it's very safe for hair.
SD A:A ratio is 20 (400/20)
So like you can see, the A:A ratio isn't a good method to know if a compound can cause hairloss or not (HD has lower A:A ratio than Epi or SD, but its for sure safer for hair than Epi or SD).
I think Androgenic ratio is a factor, but many others factors can influence in the hairloss. Probably we would must consider the RBA with the AR, if its 5a-reduced or not,...