How much does level effect resists?
 

If a level 40 bard tries to land fear on an enemy at, say level 50, how much more is he gonna get resisted? Is there a forumla?

How much benefit is there in closing that level gap?

Unsure of the formula. But you'll basically never land a spell on something that is +10. It's bad enough at +4 unless the thing is fully debuffed. Level is the single most important thing you can gain in the game, basically.

On project 1999 level is a huge factor in resisting
Note that it will never make the player completely resistant

the difference between 50 and 60 is HUGE. Even 59 and 60 is noticable.

https://wiki.project1999.com/Klandicar
is solid proof of this.
A level 59 player with 250 MR will get feared more often than a player who is level 60 with 250 MR

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On the flip side
Charm breaks feel a bit less random at 60 vs 59
Mobs resisting your spells also feels less likely at 60 vs 59 (Note)- Mobs have save vs resist too, so a mob with 500 poison resist will still resist poison spells despite your level.