Your rate (strokes/min) is not your speed, you can basically pull the same time at any rate (within reason), just depends how hard you push. As for what rate is reasonable that depends on how far your rowing and what level your at, at the gym I always see people rowing at rate 30-35, but not actually getting anywhere fast, they simply rush back up the slide to get the rate up and end up burning out or pulling abysmal splits.
As a basic idea we tend to row at 30-32 for anything 5km or less, (roughly what we would be looking at on the water in a long race), and for distance work around 21.
What exactly are you aiming for (weight loss, CV etc)?
looking at their website it seems your ergo doesn't actually give any information on how fast your going (ie time/500m), so your best bet is to get a heart rate monitor and aim to work at a set intensity, calorie estimations on these things are just that, and inaccurate at best, and wont give you a discrete value at each point through the peice, just a running total, which isn't much use for rowing at a consistent pace.