Exact Dispel ?

[Ragnarok-X]

I havent found anything and so i ask this question. I want to know about the mechanics behind dispel and or overpowering a global.
For example, if a global was put up with 50 gems and boosted by 10 gems, how much gems will a dispel need to be a sucess, and how much will it cost me to cast the same global and have it overpower the enemies ?
To what degree exactly do the casters level influence the cost of dispel and or overpowering ?
Do boosters help ? Do items like spell focus etc help ?

thanks