I couldn't view that page for some reason. I ride some pretty steep dirt roads, and have one bike that can handle 15 degrees, finally stalling out at about 20 degrees. Once past 15% I start thinking in degrees instead. 20 degrees is a blue ski run.
It won't do that grade for long though, but will climb 500 vertical feet of it without melting down. Length of the grade is a big deal though, I've melted small motors on 8% grades that were long enough.