The 5-alpha reductase inhibitory effect is seen even at commonly used doses of 160-320mg, and likely increases in a dose-dependent manner.
A few interesting reads:
http://www.jstage.jst.go.jp/pub/html/AY03S050_en.html;jsessionid=81630A849A7EB84FD9A7F5FA3482C636
"supercritical CO2 extract used in trials (Sabalselect® - a saw palmetto extract) has been tested for its actions on rat 5α-reductase where it was found that two constituent fatty acids, oleic acid and lauric acid, inhibited the activity of this enzyme with IC50 values of 54.3-54.5µg/mL and 66.2-67.6µg/mL respectively
( https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4002402/ )
An interesting read on plant derived anti-androgens:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3693613/#!po=51.3158
Many don't know that GREEN TEA can actually cause 5-alpha reductase inhibition (i.e. lowering DHT).
