Quoth MesiDoomstalker:

The real question should be, what is a useable amount of DR at each level? A linear scaling will never suffice, it will either be minimal at high level (1 level = 1 DR), or OP at low levels (starting with some base amount, then increasing as you level). So the real question is, what rate of scaling is sufficient to be worth a feat? I personally say 1:1.5, basically increasing by 2 every even level and 1 every odd level (this results in DR 30/- by level 20, which is where I'd put a good DR rating for level 20).
You just said that a linear scaling would never suffice, and then proposed a linear scaling.