Tracy, how do you figure out where to put the nest boxes so a falcon will find them?
Do you select spots that they appear to frequent?
That is certainly the first consideration and the only one the peregrines really are interested in helping with. Other consideration from the Project's perspective is that the site is secure, that a box can be installed that will be safe for the building's occupants as well as anyone below on the sidewalk/street/parking lot, that we can maintain/repair the nestbox without an inordinate amount of work, that the birds will be able to go about their normal activities without harm in the new location (not being disturbed/stressed) and that the building managment and occupants are willing to try to live with an endangered species on their roof.
What exactly makes a box have "Nest Appeal"? 
If I knew, we'd be up to our knees in appealing nests and peregrines!! Mostly its just a feel which for me I suspect is as a result of so many years of looking at what is appealing about nestsites!
We'll have to see how appealing the new "condo" is ... Jules may be a hard nut to crack when it comes to getting her off her "old" nestsite!
