How strong is DR 20/- at level 20?

[Extra Anchovies]

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).

DR 3 is still a lot at level 2, and DR 6 is just as strong at level 4. If the goal is DR 30, I'd recommend +1 per level from levels 1-10 and +2 per level from levels 11-20.