It may be different for different mobs or something. The last time I attempted to test (and this was awhile back), I used a 100 potency flubbed ID and it was netting around 50 damage without DE, and 58 or so with. That's a clean 16% damage increase which would require a base resist of ~37%. That said, RNG is a factor and I didn't do any extensive testing (because it wasn't worth the time if bosses may have different resists).

Edit: as a minor aside, a -10% additive resist debuff on 0% base resist would add 10% damage. 100 base damage - 0% * 100 versus 100 base - (-10%)*100 ... although who knows how it's calculated in detail; those are just assumptions.