Well, I have just adjusted the rate of the increase you get from using this disc in the spell file where you pointed out. Try the Ailia/Bleh spell editor from the tools section and it should be pretty straight forward on what you need to do to fix it.
Ultimately though, it seems like something probably needs to be adjusted in the source for how these percentages are handled. I know that the evasion disc is definitely not calculated correctly (even though I think it used to be months ago). I also know that some other percentages are not calculated correctly like some proc rate modifiers. I had a level 71 bard song directly from the EQLive spell list that was supposed to increase the proc rate for bard songs by only 102%, which to me means it should only be making proc rate slightly more than double what it currently is. But, this song was making bards proc almost every swing, which was extremely overpowered. I had to reduce the proc rate setting down to like 5% to get it under control.
Code:
Jonthan's Mightful Caretaker BRD/71 1: Increase Attack Speed by 70%
2: Increase All Skills Damage Modifier by 6%
4: Increase ATK by 16 (L71) to 18 (L80)
7: Increase All Skills Minimum Damage Modifier by 204%
8: Increase Proc Modifier by 102%
I can only imagine that instead of multiplying these percentage increases by X amount and then dividing by 100, it is multiplying by X amount and then not dividing at all lol. So instead of evasive increasing avoidance by 50%, it is multiplying it by 50 so you avoid 50X more often, which means you get hit maybe 2% or the time or less. And that is almost exactly what I saw from watching this evasive bug.
Hmm, maybe I can look into the source of it and just add a / 100 to the end of the calculation if I can find it. That may correct many other issues where certain things aren't balanced properly. Though, I imagine AndMetal would probably be better at finding what is going wrong here considering all of his spell and AA work lately. But, if I can find it, I think it should be a simple solution. Of course then I will need to adjust the spell values back to what they were by default to match it up with the new correction.