Question about the dark spell??

So the earthquake spell says that it cause damage to buildings and walls by %, so if I have a level 3 spell, it damages 10% (for walls efect x 4), so I need 3 spells to break any wall, because is 10 x 4 = 40%, so 2 make 80, and 3 do the job, is this how it supossed to work or i am having troubles understanding this????

Blatiever, Ecuador

4 of any level EQ spell will destroy any level walls

So for exaple if the EQ does 10% then it does 40% on walls.

Spell #1 does 40%

#2 does 24%

#3 14% and #4 finishes it off.