Ive used Natures way DIM with very good results. 2caps 2x daily for 2months, then 1cap 2x daily since. I dont believe you need 300mg a day, unless you have some major estrogen issues. For a healthy/ish male who lifts I feel the loweer dosage is better long term.(also easier to determine adjustments to dosage levels for your personal benefit)
After my first two months I started to notice a slight decrease in libido, Id say weeks 6,7, and 8, which led me to believe estrogen was to low compared to T levels. Remember estrogen is important. Also, when I say slight drop i mean slight drop, just enough to notice. Lowering the dose fixed it right up.
As far as an anti-androgen. Ive only found prostate related articles myself. Wish I could shine some light on this. However, Ive made some good strength and size gains while using it. So, IMO I feel that any low level binding of the androgen sites are offset, by far, by the favorable hormonal blance you are creating between you estrogen and testosterone. In refrence to DIM as a anti-androgen Id call this, in the big picture, a non-factor and something you shouldn't worry about.
Hope this was helpful. Good Luck!