Your are right, The way to calculate your Return in cash games is BB/100 hands or Big Blinds/hour.
ROI (Return on investment) is used for sit and go's and tournaments. I use to have a 20% ROI for mid-stake sit'n go tourneys back in 2004-2005. (ex: 5$ per 25$ sit and go in profit). The average ROI of winning players went down a lot after the golden age of poker. Top players would have a 10% ROI or so, the games became thougher.
