Not fine enough. Ideally, you want every grain cracked cleanly in half. I go with a fairly fine crush, even finer than merely cracking them in half, but I have an SS false bottom that pretty much never sticks. If you're using a braid or manifold you may run the risk of a stuck sparge.
You don't need feeler gauges or rocket science to get your crush right, just set the rollers so that they are more narrow than any piece of grain. I used to crush my grain to absolute dust until I finally got a stuck sparge. Your efficiency will suffer heavily from an insufficient crush, so better safe than sorry.