if u were asking for change the formula of ATK and MATK, there are too many you need to change on battle.c
weapon damage -> static struct Damage battle_calc_weapon_attack(struct block_list *src,struct block_list *target,uint16 skill_id,uint16 skill_lv,int wflag)
magic damage - > struct Damage battle_calc_magic_attack(struct block_list *src,struct block_list *target,uint16 skill_id,uint16 skill_lv,int mflag)
misc damage -> struct Damage battle_calc_misc_attack(struct block_list *src,struct block_list *target,uint16 skill_id,uint16 skill_lv,int mflag)
for the casting, on skill.c
int skill_vfcastfix (struct block_list *bl, double time, uint16 skill_id, uint16 skill_lv)