A good rule of thumb is four feet of space, front and back, from the wall. Fire codes require three-foot walkways, but four feet will allow you to wheel a cart in the room safely and reduce human error overall. In the future, if you have equipment to install, you may need two people in the space at a time lifting, and you want enough space to comfortably get the work done. Otherwise, your rack location will be determined by your unique space. It needs to be in a place that has the appropriate electrical requirements and ample space to run corresponding data and power cables.