The 300 fpm is actually quite wrong, but has rested unchallenged for decades. The actual downward speed of the helo has to be about 50% of its downwash speed, which is usually quite a bit faster than 300 fpm.
Most VRS/settling with power accidents are actually the problem of trying to hover out of ground effect with too little engine power, but the understanding of this is not pervasive in the community, even among accident investigators (especially when they read the books that quote 300 fpm as the VRS boundary!)
That being said, most light helos are unable to hold a 12 degree glideslope at 50 knots speed because you are in autorotation. The typical steep approach is shot by holding about 300 feet agl and slowing down to begin the descent, and the 12 degrees is not achieved until you have slowed to below about 30 knots, where the autorotative boundaries are not an issue. Most steep approaches are actually curved glideslopes that get progressively steeper, and achieve the 12 degrees as the aircraft passes over the barriers, where the steepness is needed.
Here is the web site I set up based on the last time this subject went around!
http://www.s-92heliport.com/vrs.htm