It's mostly about the crush, but there's a tradeoff between grinding into flour versus being able to run it off. Every system is a little different, but why not push the limit and see what your system can do.
Boil time and batch size also make a difference. It's easier to get higher efficiency with smaller batches because percentagewise, you can collect more runnings out of a smaller amount of grain, so you're basically "wasting" less of the malt sugars. The more wort you collect (and boil off) out of the lesser amount of grain, the higher your efficiency will be. I see this on my 3-gallon batches when I can easily hit 90% efficiency on my system. Basically, when I'm done sparging, there's not a whole lot of sugar left in the grains anymore because I still need to sparge to get enough wort in the kettle for ~4.5 gallons, then boil off a whopping ~25% of that over the course of an hour. If I do a longer boil, efficiency goes up even more because I need to sparge even more to get my preboil volume.