Arrows should be cut to where you have clearance from the rest and where you get the best spine. For that my arrow is cut 3/4" past my rest and it puts the broadhead in the front-middle of the shelf.
Mine are about 1.5" from the rest . I cut them for the best spine as well . Longer arrows are weaker in spine than a shortened arrow with the same spine deflection rating ..
I cut mine just long enough that when I pull it back the blades are in front of my knuckles. I wouldn't want to make a bad move and the blades swipe my fingers.