the formula is:
int Magic = p.getCombat().getAttacking().getLevels()[Skills.MAGIC];
int Accuracy = 140 + (3*Magic-10)/100- ((3*Magic /10 - 100)*(3*Magic /10 - 100))/100;
int Damage = 250 + (3*Magic-14)/100 - ((3*Magic /10 - 140)*(3*Magic /10 - 140))/100;
Now just use this in your combat formulas.
if(player.getequipment... == twistedbow.id)...
apply this