Leading Shots (Turret Defence Game)

Hi I am facing a problem with my turrets not hitting targets
So basically I need my turrest to lead thier shots.

So far I figured out basicly

time =(distance from turret to current enemy position)/(bullet speed)
predictedX = currentX+speedX*time
predictedY = currentY+speedY*time 			 		

Well the problem is, that with slower projectiles, its very inaccurate because the time for the projectile to reach the predicted location is different than the initial distance;time calculation

any suggestions?