Spells: Fufil's Curtailing Chant decreasing too much MR when a drum is equipped

[Lazortag]

30 Fufil's Curtailing Chant Magic based DoT (6-8 damage every 6 sec for a total of 18-24 damage), decrease MR (9-17)

http://crys.org/everquest/spells/bar...=25&maxrank=49

I cast it on myself and I lose 50 MR at level 35 (with a drum equipped), and 10 MR without a drum equipped. It seems like it's correct without a drum but it's clearly decreasing too much for when I do have a drum. I'm using a mistmoore battle drum so it should decrease 21 MR, not 50.