There aren't really "good" and "bad" levels of HR/FB%, it's a sustainability-based statistic that usually regresses to around 10.5%. It's a measure of how many of your fly balls go for homers. As a hitter, if more than 10.5% leave, you're probably getting a bit lucky, and if it's lower, probably a bit unlucky. It's the reverse for pitchers. Josh Johnson gave up something like 18% of his fly balls for homers last year, he was pretty unlucky.