If you are using your scope for rangeing the Mil-dot master is an awsome tool for a quick read, as Hungry said you need to know the size of your target to use it correctly, a must have in every ones kit.
Now wether you use Mil-dot master or Laser range finder the other toy you should have is a Scope dope from One shot Tactical to toss on your scope caps, The combination of these two tools makes for an accurate and quick rangeing and come-up solution calculator.