Make turrets deal damage equal to 30% of targets' health by default and scale accordingly

That way turrets will remain consistently a threat through the course of the game by always dealing a chunk of a target champion's total health as percentage damage that scales accordingly (e.g if you have maximum 1000 health the turret will deal 300 damage against you and if you have 2000 health it will deal 600 damage on you meaning that turrets will always kill you in four shots assuming you're in full health).