It's linear up to 75% and then it's a steep nosedive in efficiency. 100% resistance is only worth 80%. 200% resistance is only worth 88.99%. However it's still good to shred resistances from 75% down to 0%, that's 4x damage. Unless you can gain 333% physical damage bonus, it's better to shave 70% physical DR off that ruin guard.
Once you’re at World Level 5, the lowest you will ever be able to reduce it will be to World Level 4. Lower your Genshin Impact World Level by following these steps: Open the Paimon Menu. Select the ‘i’ icon next to World Level (just above the birthday) Select ‘Lower World Level’ and ‘Confirm Adjustment’. Once your World Level isWorld level increase/decrease only through katheryn in mond or liyue and immediate boss resetting on leaving the area. Quest pause. Can’t progress further if you aren’t at the required world level
Suppose a level 80 enemy has 200 defense. The damage reduction will be Def/ (Def+ 5x level+500)= 200/ (200+400+500)=0.22 or 22% . So it means that the amount of damage you would have done if enemy defense and level were 0 would be now reduced by 22%. Hence the lower the defence lower the damage resisting capacity because the defence formula is LqmU.