My understanding is that the key figure is actually the "drag factor" and that the damper setting is going to vary substantially from machine to machine. Damper at 3 on one machine could have same drag factor as a damper setting of 7 on another machine. And further, depending on your weight or preferences or skill or whatever, you want the drag factor somewhere between 100 and 150.
Anyway. Here's a link to the C2 site.
http://www.concept2.co.uk/guide/guid...e=damper_lever
edit: I see it looks like the range is actually 130-140 in order to simulate rowing on the water. The point is that more/harder isn't better.