Thats a great number for your efficiency. It can vary depending on the source of your grains and the crush they recieve, but I usually get anywhere from 70% to 80%, depending on the size of the recipe, and the store where I purchase my grains.
The finer the crush, the better the effeciency, but if it gets too fine it will start to cause problems. You may find that it gets hard to sparge, and you may get astingent tannins from the grain husks being too finely crushed.
There are other things that can affect effeciency such as mash tun design, sparge water temp, sparge duration etc., but I would be happy with what you have gotten thus far.