Well, as you cut them down, you are essentially changing the spine, so you really only want to go so far, otherwise the more you cut off, the stiffer you're making the arrow (assuming all other variables stay the same). Other than that, I just wouldn't want them so short that I risk the arrow popping off the back of the rest, or getting my bow hand too close to my broadheads. I shoot 27.5" arrows with a 28.5" DL. It's just fine for me.