How to set a variable constantly

How do I constantly start setting a variable forever while the player is in the world? I'd think you'd use a while true block but when I tried to do that once my game instantly crashed upon loading into the world. The thing I want to do is to make a variable to shorten the Look X position [player] with raytrace distance... block because it's laggy to scroll to the bottom of the entity data tab and get the block from there.

(Just want something to make it easier to make stuff)