The .8 figure sounds about right, although you also have to use a little common sense in some cases. For instance, I was playing in a low-magic game a few months ago, we were all 5th level I believe, and had only a few thousand gp worth of items between the six of us. We then fight a Flesh Golem. Well, the DM had told me at the start of this campaign that since magic is so rare, he'd allow any magic weapon to penetrate any and all DR. So of course, I cast Magic Weapon on the fighter's axe, thinking it will allow him to hurt the golem. But nope. The guy expected us to have adamantine weapons, even though we couldn't have afforded them if we'd tried. It turned into nearly a TPK, since we basically couldn't damage the thing. So the point of this roundabout anecdote is that some monsters and challenges are MUCH harder without magic, whereas most are only moderately more difficult. As long as you're careful of the first set, things should go fine.