That's not totally unreasonable. Without crunching the numbers, I'm assuming your OG is around 1.100? The brown sugar will help keep the FG low. Attenuation tends to drop off with higher gravity beers, but 75% shouldn't be too hard to get with "normal" brewing practices.
That's a good link if you ever want to get better attenuation out of bigger beers. Remember to ferment at as low of a temp as the yeast will tolerate. Bigger beers show off fermentation flaws more than regular strength beers.